Semiconductor laser device, and method of manufacturing the same

ABSTRACT

A semiconductor laser device comprises a laminate consisting of a semiconductor layer of first conductivity type, an active layer and a semiconductor layer of second conductivity type, which is different from the first conductivity type, that are stacked in order, with a waveguide region being formed to guide a light beam in a direction perpendicular to the direction of width by restricting the light from spreading in the direction of width in the active layer and in the proximity thereof, wherein the waveguide region has a first waveguide region and a second waveguide region, the first waveguide region is a region where light is confined within the limited active layer by means of a difference in the refractive index between the active layer and the regions on both sides of the active layer by limiting the width of the active layer, and the second waveguide region is a region where the light is confined therein by providing effective difference in refractive index in the active layer.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor laser device havingstripe ridge being formed. More particularly, the present inventionrelates to a semiconductor laser device which uses GaN, AlN or InN, orthe Group III-V nitride compound semiconductor (In_(b)Al_(d)Ga_(1-b-d)N,0≦b, 0≦d, b+d<1) that is a mixed crystal of the compounds describedabove.

Description of Related Arts

Recently, nitride semiconductor laser devices have been receivingincreasingly demands for the applications in optical disk systems suchas DVD which are capable of recording, and reproducing a large amount ofinformation with a high density. Accordingly, vigorous research effortsare being made in the field of nitride semiconductor laser device.Because of the capability to oscillate and emit visible light over abroad spectrum ranging from ultraviolet to red, the nitridesemiconductor laser device is expected to have wide applications such aslight sources for laser printer and optical network, as well as theoptical disk system.

With regard to the structure of the laser device, in particular, variousresearches have been made and a number of proposals have been made forthe structure that enables preferable control of transverse oscillationmode. Among these, ridge waveguide structure is viewed as promising, andis employed in the nitride semiconductor laser device that was shippedfirst in the world.

The ridge waveguide structure for a semiconductor laser device makes iteasier to drive laser oscillation due to the simple structure, althoughvariations are likely to occur in the characteristics of the devicesduring volume production. This is because the characteristics are causedto vary by variations in the dimensions of mesa stripe in the case ofridge waveguide structure, while the dimensional accuracy of the mesastripe is determined by the accuracy of etching and the dimensionalaccuracy of the mesa stripe cannot be made higher than the accuracy ofetching. In the case of a semiconductor laser device made from asemiconductor material which is likely to suffer significant etchingdamage in the active layer or damage caused by exposing the active layersurface to the etching atmosphere, laser characteristics degrade due tothe etching damage in the active layer and on the active layer surfacewhen a semiconductor laser device of perfect refractive index guidedtype is made by etching deeper than the active layer thereby formingridges. Therefore, such a semiconductor laser device must be made in theeffective refractive index type waveguide structure wherein stripes areformed to a depth that does riot reach the active layer. However, in thecase of effective refractive index type waveguide structure, variationsin the device characteristics due to the variation in the stripeconfiguration mentioned above become significant, thus resulting inconsiderable variations in the device characteristics during volumeproduction.

In order to apply the nitride semiconductor laser device in the fieldsdescribed previously, it is indispensable to provide a device which canbe mass-produced with stable quality.

However, the structure of the laser devices known at present has abottle neck in the formation of the ridge waveguide. This is because,while the ridge waveguide is formed by growing nitride semiconductorthat constitutes the device, then removing a part of the nitridesemiconductor by etching the upper layer thereby forming the ridge whichconstitutes the waveguide, accuracy of the etching has a great effect onthe characteristics of the laser device obtained as mentionedpreviously. That is, since the transverse mode is controlled by theconfiguration, particularly height and width, of the ridge thatconstitutes the ridge waveguide and the far field pattern (F.F.P.) ofthe laser beam is determined accordingly, an error in the control of thedepth of etching when forming the ridge waveguide is a major factorwhich directly causes variations in the device characteristics.

Dry etching techniques such as reactive ion etching (RIE) have beenknown for etching nitride semiconductor, but it has been difficult tocontrol the depth of etching to such an accuracy as to completely solvethe problem of variations in the device characteristics with theseetching techniques.

Design of devices in recent years in a trend to have a multitude oflayers that are controlled to be several atoms in thickness formed inthe device, such as in the case of super lattice structure. This alsocontributes to the variations in the device characteristics caused bythe etching accuracy. Specifically, when forming the layers whichconstitute the device structure, the layers are formed with an extremelyhigh accuracy and it is difficult to achieve the device structure ofsophisticated design by forming the ridge and other structure with theetching technique having an accuracy lower than the accuracy of filmforming by several orders of magnitude, thus making an obstacle to theimprovement of device characteristics.

For example, when forming a nitride semiconductor laser device having ahigh output power in the refractive index guiding type structure whereridge waveguide is provided on the active layer without etching theactive layer, accuracy of etching depth must be controlled so as to keepthe effective difference in refractive index between a portion of theactive layer right below the ridge and other portion of the active layerto one hundredth. In order to achieve this accuracy, the ridge must beformed by etching while controlling the depth with an accuracy within0.01 μm till a very small portion of p-type cladding layer remains, incase the layer right above the active layer is the p-type claddinglayer. On the other hand, width of the ridge waveguide may have loweraccuracy but must be etched with an accuracy of 0.1 μm.

When the RIE process is employed for etching the nitride semiconductor,the layer exposed by etching and the surface thereof are prone todamage, which leads to deterioration of the device characteristics andreliability. Etching can be done in a wet etching process as well as adry etching process, although wet etching solution which is applicableto nitride semiconductors has not been developed.

As described above, whether nitride semiconductor laser device havinghigh functionality can be made or not in volume production with lessvariations in the characteristics heavily depends on the accuracy offorming the ridge, waveguide in the etching process, and, it hascritical importance to form the ridge waveguide with a high accuracy.

In light of the circumstances described above, the present inventorshave invented a laser device or an end face light emission device and amethod for manufacturing the same which, even in the case of asemiconductor laser device of stripe configuration and despite thesemiconductor laser device has a resonator of excellent oscillation andwave guiding characteristics, allows stable control of transverse modeand is capable of emitting laser beam of excellent F.F.P., with lessvariations in the device characteristics even when mass-produced.

SUMMARY OF THE INVENTION

An object of the present invention can be achieved with thesemiconductor laser device of the present invention having such aconstitution as described below.

A first semiconductor laser device of the present invention comprises alaminate consisting of a semiconductor layer of first conductivity type,an active layer and a semiconductor layer of second conductivity type,which is different from the first conductivity type, that are stacked inorder, with a waveguide region being formed to guide a light beam in adirection perpendicular to the direction of width by restricting thelight from spreading in the direction of width in the active layer andin the proximity thereof, wherein the waveguide region has a first,waveguide region and a second waveguide region, the first waveguideregion is a region where light is confined within the limited activelayer by means of a difference in the refractive index between theactive layer and the regions on both sides of the active layer bylimiting the width of the active layer, and the second waveguide regionis a region where the light is confined therein by providing effectivedifference in refractive index in the active layer.

In the first semiconductor laser device of the present inventionconstituted as described above, since the waveguide region has the firstwaveguide region where light is confined within the active layer byactually providing a difference in the refractive index between theactive layer and the regions on both sides of the active layer,oscillation in the transverse mode can be more surely suppressed in thefirst waveguide region and the beam can be controlled reliably therebyemitting laser beam having excellent F.F.P.

Also in the first semiconductor laser device which has the secondwaveguide region constituted by forming a region that has effectivelyhigh refractive index in the active layer, since the waveguide can beformed without exposing the active layer that functions as the waveguidedirectly to the outside in the second waveguide service life of thedevice can be prolonged and reliability can be improved. Thus the firstsemiconductor laser device of the present invention has the features ofthe first waveguide region and the second waveguide region combined.

In the first semiconductor laser device of the present invention, theactive layer in the first waveguide region can be constituted by forminga first ridge that includes the active layer thereby limiting the widthof the active layer, and the region having effectively higher refractiveindex can be constituted by forming a second ridge in the layer of thesecond conductivity type.

Also in the first semiconductor laser device of the present invention,the first ridge can be formed by etching both sides of the first ridgetill the layer of the first conductivity type is exposed and the secondridge can be formed by etching both sides of the second ridge so thatthe layer of the second conductivity type remains on the active layer.

In the first semiconductor laser device of the present invention,thickness of the layer of the second conductivity type located on theactive layer on both sides of the second ridge is preferably 0.1 μm orless, in which case it is made possible to more surely control thetransverse mode.

Further in the first semiconductor laser device of the presentinvention, the second ridge is preferably longer than the first ridge,in which case the reliability can be improved further.

Further in the first semiconductor laser device of the presentinvention, the first waveguide region a preferably includes oneresonance end face of the laser resonator in which case laser beam ofexcellent F.F.P. can be obtained.

Also in the first semiconductor laser device of the present invention,it is preferable to use the one resonance end face as the light emittingplane, in which case laser beam of more excellent F.F.P. can beobtained.

In the first semiconductor laser device of the present invention, lengthof the first waveguide region is preferably 1 μm or more.

Further in the first semiconductor laser device of the presentinvention, the semiconductor layer of the first conductivity type, theactive layer and the semiconductor layer of the second conductivity typecan be formed from nitride semiconductor.

Also in the semiconductor laser device described above, the active layercan be constituted from a nitride semiconductor layer which includes In,in which case the laser can be oscillated in the visible region ofrelatively short wavelength and in the ultraviolet region.

In the first semiconductor laser device of the present invention, it ispreferable to form insulation films on both sides of the first ridge andon both sides of the second ridge, while the insulation film is made ofa material selected from the group consisting of oxides of Ti V, Zr, Nb,Hf and Ta and compounds SiN, BN, SiC and AlN.

A second semiconductor laser device of the present invention comprises alaminate which consists of a layer of the first conductivity type, anactive layer and a layer of the second conductivity type that isdifferent from the first conductivity type being stacked in order, andis provided with a stripe waveguide region, wherein the stripe waveguideregion has at least a first waveguide region C₁ in which a stripe-shapedwaveguide based on absolute refractive index is provided and a secondwaveguide region C₂ in which a stripe-shaped waveguide based oneffective refractive index is provided, which are arranged in thedirection of the resonator. In this constitution, since the laser deviceof the present invention has the second waveguide region C₂ havingexcellent device reliability and the first waveguide region C₁ havingexcellent controllability of the transverse oscillation and excellentbeam characteristic, the laser device combines both of thesecharacteristics thus making it possible to provide various laser devicesaccording to the application without tedious modification of the devicedesign. In the effective refractive index type waveguide, a stripe ridgeformed in the layer of the second conductivity type located on theactive layer makes it possible to keep the active layer remain in thestate of growing, so that the waveguide does not deteriorate whenoperating the device, thus ensuring excellent reliability of the device.Also because the first waveguide region C₁ of refractive index guidingtype is provided in the waveguide by etching deeper than the activelayer thereby creating a difference in the refractive index on bothsides of the waveguide region, the transverse mode can be easilycontrolled. Providing this as the waveguide of the laser device makes itpossible to easily change the transverse mode in the waveguide. In thisspecification, the waveguide which has the first waveguide region willbe referred to as total refractive index type waveguide or absoluterefractive index type waveguide in order to avoid confusion with theeffective refractive index type waveguide.

In the second semiconductor laser device of the present invention, theabsolute refractive index of the first waveguide region C₁ is achievedby means of the stripe ridge which is provided so as to include thelayer of the first conductivity type, the active layer and the layer ofthe second conductivity type, and the effective refractive index of thesecond waveguide region C₂ is achieved by means of the stripe ridgewhich is provided in the layer of second conductivity type. With thisconstitution, since the first waveguide region C₁ and the secondwaveguide region C₂ can be formed easily in the laser device, laserdevices of diverse characteristics can be made by simple design.

A third semiconductor laser device of the present invention comprises alaminate which consists of a layer of the first conductivity type, anactive layer and a layer of the second conductivity type that isdifferent from the first conductivity type being stacked in order, andis provided with a waveguide region of stripe configuration, wherein thestripe waveguide region has at least a second waveguide region where aportion of the layer of the second conductivity type is removed and astripe ridge is provided in the layer of the second conductivity type,and a first waveguide region C₁ where portions of the layer of secondconductivity type, the active layer and the layer of first conductivitytype are removed and a stripe ridge is provided in the layer of thefirst conductivity, type, which are arranged in the direction ofresonator. With this constitution, since the stripe waveguide region isconstituted from the region (first waveguide region C₁) where a part ofthe active layer is removed and the region (second waveguide region C₂)where the active layer is not removed, damage to the active layer casedby the removal can be restrained within a part of the waveguide, therebyimproving the reliability of the device. For a semiconductor materialwhich is heavily subject to damage, deterioration in the reliability andcharacteristic of the device caused by the partial removal of the activelayer, a laser device having desired reliability and characteristic ofthe device can be achieved by designing the proportion occupied by thefirst waveguide region C₁, since the first waveguide region C₁ isprovided only partially. Also by changing the length of (proportion ofthe waveguide constituted from) and location of the first waveguideregion C₁ and the second waveguide region C₂, laser devices of variouscharacteristics can be made and, particularly, laser devices havingdesired beam characteristics can be easily obtained.

In the second and third semiconductor laser devices, the first waveguideregion C₁ and the second waveguide region C₂ may also be constituted byremoving a part of the laminate structure and forming a ridge waveguidecomprising a stripe ridge. With this constitution, laser devices ofridge waveguide structure comprising the stripe ridge having diversecharacteristics can be made.

In the second and third semiconductor laser devices, it is preferable tomake the stripe of the second waveguide region C₂ longer than the firstwaveguide region C₁. With this constitution, a laser device havingexcellent reliability can be made from a semiconductor material whichundergoes greater deterioration due to the formation of the firstwaveguide region C₁, for example a semiconductor material which isdamaged when a part of the active layer is removed or exposed to theatmosphere.

Also in the second and third semiconductor laser devices, it ispreferable that at least one of the resonance end faces of thesemiconductor laser device is formed at the end of the first waveguideregion C₁. With this constitution, by providing the first waveguideregion C₁ having excellent controllability of the transverse mode, onone of the resonance end faces, guiding of light can be controlled moreeffectively than in the case of providing the first waveguide region C₁at other position, thereby making it possible to obtain laser deviceshaving diverse characteristics.

Also in the second and third semiconductor laser devices, it ispreferable that the resonance end face formed on the end of the firstwaveguide region C₁ is the light emitting plane. With this constitution,by providing the first waveguide region C₁ which has excellentcontrollability of transverse mode on the laser beam emitting plane,beam characteristic can be directly controlled and a laser device havingdesired F.F.P. and laser beam aspect ratio can be obtained.

Also in the second and third semiconductor laser devices, it ispreferable that length of the stripe of the first waveguide region C₁which has the resonance end face on the end face thereof is preferably 1μm or longer. With this constitution, more reliable control of F.F.P.and laser beam aspect ratio can be achieved and the laser devices ofless variations in the characteristics are obtained.

The second and third semiconductor laser devices may also be constitutedby using a nitride semiconductor in the layer of the first conductivitytype, the active layer and the layer of the second conductivity type.This constitution makes it possible to make laser devices having diversecharacteristics from the nitride semiconductor in which it is difficultto form a buried structure of regrowth layer by ion implantation. Sincethe service life of the device becomes significantly shorter when a partof the active layer is removed by etching or the like in nitridesemiconductor, it has been difficult to commercialize a laser devicecomprising total refractive index type waveguide in which a part of theactive layer is removed. However, since a part of the waveguide becomesthe first waveguide region C₁, a laser device having excellentcontrollability of the transverse mode can be made while keeping thedevice life from decreasing.

In the second and third semiconductor laser devices, the active layermay also be constituted from a nitride semiconductor laser whichincludes In. With this constitution, a laser device which oscillatesover a range of wavelengths from ultraviolet to visible light can bemade.

Also in the second and third semiconductor laser devices, the firstwaveguide region C₁ may include n-type nitride semiconductor and thesecond waveguide region C₂ may include p-type nitride semiconductor.

Also in the second and third semiconductor laser devices, it ispreferable that the second waveguide region C₂ has a p-type, claddinglayer which includes p-type nitride semiconductor and the stripe ridgeof the second waveguide region is formed while keeping the thickness ofthe p-type cladding layer is less than 0.1 μm. With this constitution, alaser device having low threshold current and excellent controllabilityof the transverse mode can be made. Here thickness of the p-typecladding layer refers to the distance between the exposed surface of thep-type cladding layer in a region where the ridge is not formed and theinterface with the adjacent layer below the p-type cladding layer, and“above the active layer” means the location above the interface betweenthe active layer and the adjacent layer located above. That is, in casethe active layer and the p-type cladding layer are provided in contactwith each other, the exposed surface mentioned above is formed at adepth in the p-type cladding layer where it remains with a thicknessgreater than 0 and within 0.1 μm. In case a guide layer or the like isprovided between the active layer and the p-type cladding layer as inthe case of the first embodiment to be described later, the exposedsurface mentioned above is formed above the interface between the activelayer and the adjacent layer located above, and below a depth in thep-type cladding layer where it remains with a thickness of 0.1 μm or ina layer between the active layer and the p-type cladding layer.

The second and third semiconductor laser devices may also have such aconstitution as the nitride semiconductor is exposed on the side facesof the stripe ridge of the first waveguide region C₁ and on the sidefaces of the stripe ridge of the second waveguide region C₂, aninsulation film is provided on the side face of the stripe ridge, andthe insulation film is made of a material selected from the groupconsisting of oxides of at least one element selected from Ti, V, Zr,Nb, Hf and Ta and at least one kind of compounds SiN, BN, SiC and AlN.With this constitution, satisfactory difference of refractive index canbe provided in the stripe ridge of the nitride semiconductor laserdevice, and the laser device having the stripe waveguide region ofexcellent controllability of the transverse mode can be made.

In the second and third semiconductor laser devices, width of the striperidge is preferably in a range from 1 μm to 3 μm. With thisconstitution, the stripe waveguide region of excellent controllabilityof the transverse mode can be formed within the waveguide layer in thefirst waveguide region C₁ and the second waveguide region C₂, thusachieving a laser device free of kink in the current optical outputcharacteristic.

A method for manufacturing the semiconductor laser device of the presentinvention achieves the object of the present invention in a constitutiondescribed below.

The method for manufacturing the semiconductor laser device of thepresent invention comprises a laminating process in which the layer ofthe first conductivity type, the active layer and the layer of thesecond conductivity type are stacked in order by using nitridesemiconductor to form a laminate, a process of forming a firstprotective film of stripe configuration after forming the laminate, afirst etching process in which the laminate is etched in a portionthereof where the first protective film is not formed thereby to formthe stripe ridge in the layer of the second conductivity type, a secondetching process in which a third protective film is formed via the firstprotective film on a portion of the surface which has been exposed inthe first etching process and the laminate is etched in a portionthereof where the third protective film is not formed thereby to formthe stripe ridge in the layer of first conductivity type, a process inwhich a second protective film having insulating property made of amaterial different from the first protective film is formed on the sideface of the stripe ridge and on the nitride semiconductor surfaceexposed by etching, and a process of removing the first protective filmafter the second protective film has been formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view schematically showing the constitution ofthe laser device according to an embodiment of the present invention.

FIG. 1B is a sectional view of the second waveguide region of the laserdevice of the embodiment.

FIG. 1C is a sectional view of the first waveguide region of the laserdevice of the embodiment.

FIG. 2A is a schematic sectional view prior to forming the ridge in thelaser device of the prior art.

FIG. 2B is a schematic sectional view after forming the ridge in thelaser device of the prior art.

FIG. 2C is partially enlarged view of a part denoted “a” in FIG. 2B.

FIG. 2D is partially enlarged view of a part denoted “b” in FIG. 2B.

FIG. 3A is a perspective view schematically showing the constitution oflayers in the laser device according to an embodiment of the presentinvention, and FIG. 3B is a side view of FIG. 3A.

FIG. 4A is a side view of the laser device of a variation according tothe present invention.

FIG. 4B is a side view of the laser device of another variationaccording to the present invention.

FIG. 5A through FIG. 5D are perspective views showing the process offorming the ridge of the laser device of the present invention.

FIG. 5E is a sectional view of a portion where the second waveguideregion of FIG. 5C is to be formed.

FIG. 5F is a perspective view of a portion where the second waveguideregion of FIG. 5D is to be formed.

FIG. 6A through FIG. 6C are perspective views showing the process offorming the ridge of the laser device of the present invention by amethod different from the method shown in FIG. 5A through FIG. 5D.

FIG. 7A through FIG. 7D are perspective views showing the process offorming the electrodes in the laser device of the present invention.

FIG. 8 is a schematic sectional view of the second waveguide region ofthe laser device according to the first embodiment of the presentinvention.

FIG. 9 is a schematic sectional view of the first waveguide region ofthe laser device according to the first embodiment of the presentinvention.

FIG. 10 is a graph showing the acceptance ratio as a function of thedepth of etching in the laser device of effective refractive index type.

FIG. 11 is a graph showing the drive current as a function of the depthof etching in the laser device of effective refractive index type.

FIG. 12 is a graph showing the service life as a function of the depthof etching in the laser device of effective refractive index type.

FIG. 13A is a perspective view of the laser device according to thesixth embodiment of the present invention.

FIG. 13B is a cross sectional view of the laser device according to thesixth embodiment of the present invention.

FIG. 14A is a perspective view of the laser device according to theseventh embodiment of the present invention.

FIG. 14B is a sectional view of the second waveguide region of the laserdevice according to the seventh embodiment of the present invention.

FIG. 14C is a sectional view of the first waveguide region of the laserdevice according to the seventh embodiment of the present invention.

FIG. 15A is a perspective view of the laser device according to theeighth embodiment of the present invention.

FIG. 15B is a cross sectional view of the laser device according to theeighth embodiment of the present invention.

FIG. 16A through FIG. 16D are perspective views showing the method formanufacturing the laser device of the present invention by using devicesformed on a wafer.

FIG. 17A and FIG. 17B are schematic sectional, views showing the cuttingposition according to the method for manufacturing the laser device ofthe present invention.

FIG. 18 is a schematic diagram showing the process of forming thereflector film according to the method for manufacturing the laserdevice of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now the semiconductor laser device of the present invention will bedescribed below by way of preferred embodiments with reference to theaccompanying drawings.

The semiconductor laser device of an embodiment according to the presentinvention has a first waveguide region C₁ and a second waveguide regionC₂ as stripe waveguide region as shown in FIG. 1A.

The first waveguide region C₁ is a waveguide region where a ridge (firstridge 201) is formed so as to include an active layer 3 and a differencein the refractive index is created between the active layer 3 and theregions (in the atmosphere in this case) located on both sides thereofas shown in FIG. 1C, thereby to confine light within the active layer 3.In this specification, the waveguide region where light is confined byproviding an actual difference in the refractive index between theactive layer and the regions on both sides thereof will be referred toas the total refractive index type waveguide.

The second waveguide region C₂ is a waveguide region where a ridge(second ridge 202) is formed in the semiconductor layer located on theactive layer so that the effective refractive index of the active layer3 located below the second ridge 202 is made higher than that of theactive layer located on both sides thereof as shown in FIG. 1B, therebyto confine light within the active layer 3 having higher effectiverefractive index. In this specification, the waveguide region wherelight is confined by providing an effective difference in the refractiveindex between the active layer and the regions on both sides there ofwill be referred to as the effective refractive index type waveguide.

The semiconductor laser according to the present invention ischaracterized by the total refractive index type waveguide and theeffective refractive index type waveguide provided in the waveguide.

Specifically, the second waveguide region C₂ is constituted by formingthe laminate consisting of the layer of the first conductivity type, theactive layer and the layer of the second conductivity type which isdifferent from the first conductivity type being stacked one on another,and forming the second stripe ridge 202 on the layer 2 of the secondconductivity type to such a depth as the active layer is not reached,and the first waveguide region C₁ is constituted by forming the firststripe ridge 201 so as to include portions of the layer 2 of the secondconductivity type, the active layer 3 and the layer 1 of the firstconductivity type.

According to the present invention, by having the first waveguide regionC₁ and the second waveguide region C₂ in the waveguide as describedabove, semiconductor laser devices of diverse characteristics can beobtained. In the semiconductor laser device of the present invention,the waveguide having the first waveguide region C₁ and the secondwaveguide region C₂ can be formed in various forms as shown in FIGS. 3and 4. FIG. 3A is a partially cutaway perspective view of the laserdevice of such a structure as the stripe ridge is formed by removing apart of the laminate. FIG. 3B is a cross section viewed in the directionof arrow in FIG. 3A. FIGS. 4A and 4B show a waveguide structuredifferent from that shown in FIG. 3.

According to the present invention, as shown in FIGS. 3 and 4, variousconstitutions can be employed where the first waveguide region C₁ andthe second waveguide region C₂ are disposed in various arrangements inthe resonator direction (longitudinal direction of the stripe ridge).The semiconductor laser according to the present invention may also havea waveguide region other than the first waveguide region C₁ and thesecond waveguide region C₂, as a matter of fact. For example, awaveguide region 203 different from the first waveguide region C₁ andthe second waveguide region C₂ may be provided between the firstwaveguide region C₁ and the second waveguide region C₂ as shown in FIG.4A. FIG. 3 shows such a structure as the first waveguide region C₁ isprovided so as to include one of the resonance end faces of theresonator and the second waveguide region C₂ is provided so as toinclude the other resonance end face. FIG. 4A shows a semiconductorlaser device having such a structure as the first ridge 201 whichconstitutes the first waveguide region C₁ and the second stripe ridge202 which constitutes the second waveguide region C₂ are joined via awaveguide region 203 which is formed so as to incline with respect tothe vertical direction (perpendicular to the resonator direction). Thusthe first waveguide region C₁ and the second waveguide region C₂ may beformed either substantially continuously in the resonator direction asshown in FIG. 3 or with another region being interposed therebetween asshown in FIG. 4A.

According to the present invention, it is not necessary that width ofthe first ridge 201 and width of the second ridge 202 are substantiallythe same. For example, in case the side face of each ridge is formed toincline as shown in FIGS. 1 and 3, width at the base of the first ridge201 provided to constitute the first waveguide region C₁ and width atthe base of the second ridge 202 provided to constitute the secondwaveguide region C₂ become inevitably different from each other. Theside face of the first ridge and the side face of the second ridgepreferably lie in the same plane. While the stripe ridges shown in FIG.1 and FIG. 3 are formed in the normal mesa configuration where the sidefaces are inclined so that width decreases from the base to the top, theridge may also be formed in the inverted mesa configuration where thewidth increases from the base to the top, and further both side faces ofthe mesa may be inclined either in the same way or in the oppositemanner.

Width of the top surface of the first ridge 201 and width of the topsurface of the second ridge 202 may be different from each other.Further, width of the first ridge 201 and width of the second ridge 202viewed in the horizontal section may be different so as to changediscontinuously at the border of the first ridge 201 and the secondridge 202.

[Resonator Structure]

In the semiconductor laser device of this embodiment, the stripewaveguide is constituted by removing a part of the laminate structureand forming the ridge. That is, as shown in FIGS. 1 and 3, the resonatorhas such a structure as the stripe ridge is formed by removing bothsides of a portion which would become the ridge by etching or othermeans in the laminate consisting of the layer 1 of the firstconductivity type, the active layer 3 and the layer 2 of the secondconductivity type, which is suited to the so-called ridge waveguidelaser device. According to the present invention, since at least thefirst waveguide region C₁ and the second waveguide region C₂ areprovided by means of the stripe ridge, beam characteristic can beimproved and particularly F.F.P. can be controlled in a desired shapefrom ellipse to true circle, so that various laser devices havingdiverse characteristics can be provided. The stripe ridge is not limitedto the normal mesa configuration shown in FIGS. 1 and 3 as describedabove, and may be formed in inverted mesa configuration or in stripeshape having vertical side faces. That is, the ridge shape may bechanged according to the laser characteristic required.

Also in the semiconductor laser device of the present invention, theridge may be buried by regrowing crystal on both sides of the ridgeafter forming the stripe ridges 201, 202 when constituting the firstwaveguide region C₁ and the second waveguide region C₂.

As described above, since the present invention assumes the ridgewaveguide structure having the stripe ridge, it is made possible notonly to achieve production at a lower cost but also to make laserdevices having diverse characteristics by arranging the first waveguideregion C₁ and the second waveguide region C₂ in various combinations inthe waveguide. For example, since it is made possible to control thebeam characteristic, satisfactory F.F.P. can be achieved without usingbeam correction lens or the like.

In the laser device of the present invention, the first and secondstripe ridges 201, 202 provided in the first waveguide region C₁ and thesecond waveguide region C₂ have such a configuration as shown in FIG. 1Band FIG. 1C.

The present invention is also applicable to devices other than laseroscillation device, for example end face light emitting devices such aslight emitting diode. The device having the constitution shown in FIG. 1can be operated as a light emitting diode by driving the device belowthe threshold of oscillation, and a device which emits light from an endface without laser oscillation can be obtained by inclining thewaveguide from the direction which is perpendicular to the end face,rather than making the waveguide perpendicular to the end face.

[Laminate Structure]

Now the structure of the laminate consisting of the layer of firstconductivity type, the active layer and the layer of second conductivitytype provided in the semiconductor device of this embodiment will bedescribed in detail below.

In the semiconductor device of this embodiment, as shown in FIG. 1,cladding layers 5, 7 are provided in the layer 1 of first conductivitytype and the layer 2 of second conductivity type, respectively, andlight is confined in the direction of thickness by sandwiching theactive layer 3 with the cladding layers 5, 7. Thus the optical waveguideregion is provided within the laminate where light is confined in thewidth direction (perpendicular to, the direction of thickness andperpendicular to the direction of resonance) by means of ridge and alsolight is confined in the direction of thickness by means of the claddinglayers 5, 7. In the semiconductor laser device of the present invention,various kinds of semiconductor material known in the prior art can beused such as those based on, for example, GaAlAs, InGaAsP and GaAlInN.

In the semiconductor laser device of the present invention, the stripewaveguide region is formed in correspondence to the ridge in the activelayer between the layer of the first conductivity type and the layer ofthe second conductivity type, and in the vicinity thereof, while thelongitudinal direction of the stripe and the direction of lightpropagation are substantially identical. That is, while the stripewaveguide region is constituted mainly from the active layer in whichlight is confined, part of light is guided while spreading in thevicinity thereof, and therefore a guide layer may be formed between theactive layer and the cladding layer so that the region including theguide layer is used as the optical waveguide layer.

[Second Waveguide Region C₂]

The second waveguide region C₂ of the present invention is a regionprovided as the effective refractive index type waveguide in thewaveguide of the semiconductor laser device. Specifically, the striperidge 201 is formed in the layer 2 of second conductivity type 2 locatedon the active layer 3 of the laminate, and the stripe waveguide regionis formed by providing effective difference in refractive index in thedirection of plane (width direction) of the active layer.

In a laser device of effective refractive index type of the prior art inwhich the waveguide consists of the second waveguide region C₂ only, thestripe ridge 202 is formed by etching using a mask 20 after forming thesemiconductor layers as shown in FIG. 2. Since the stripe ridge 202 isformed by etching to such a depth that does not reach the active layerthereby to provide the effective difference in refractive index in theactive layer (waveguide layer), characteristics of the device varysignificantly depending on the width Sw of the stripe, height of theridge (depth of stripe) Sh₁ and distance Sh₂ between the surface exposedby etching and the top plane of the active layer as shown in FIG. 2B.These factors cause serious variations in the device characteristicsduring production thereof. That is, the variations in the devicecharacteristics are caused directly by error Hd in the height of theridge (depth of stripe) and error Wd in the width of the stripe relatedto the accuracy of etching shown in FIG. 2C and FIG. 2D. This is becausethe waveguide region formed in the active layer (waveguide layer) isprovided by making use of the effective difference in refractive indexcorresponding to the ridge 202 by means of the stripe ridge 202 providedin the active layer (waveguide layer), and therefore the configurationof the ridge has a significant influence on the effective difference inrefractive index. The error Hd in the height of the ridge is also theerror in the distance between the surface exposed by etching and the topplane of the active layer. When the distance Sh₂ between the top planeof the active layer and the surface exposed by etching is too large, theeffective difference in refractive index becomes smaller resulting insignificant influences on the device characteristics such asinsufficient confinement of light. As described above, since theeffective refractive index is dependent on the distance Sh₂ between thetop plane of the active layer and the surface exposed by etching,variations in the distance cause variations in the effective refractiveindex.

FIGS. 10, 11 and 12 show the ratio of products which pass the etchingdepth inspection, drive current and service life for the laser device ofthe effective refractive index type of the prior art. As will beunderstood from the drawings, characteristics of the laser device arevery sensitive to the depth of etching.

In the laser device of the present invention, since the second waveguideregion C₂ formed by etching to such a depth that does not reach theactive layer is provided as a part of the waveguide, the active layer isprevented from being damaged by etching in the second waveguide regionC₂, and therefore reliability of the device can be improved. In the caseof a material which undergoes significant device characteristics whenthe active layer, is exposed to the atmosphere, providing the secondwaveguide region C₂ makes it possible to restrict the reliability of thedevice from deteriorating.

[First Waveguide Region C₁]

According to the present invention, laser devices of variouscharacteristics can be easily made by forming the first waveguide regionC₁ in addition to the second waveguide region C₂ as the stripe waveguideregion, as described previously. This is an effect brought about by theexcellent controllability of the transverse mode of the first waveguideregion C₁ which is made by forming the stripe ridge 201 that includesportions of the active layer and the layer 1 of the first conductivitytype in the laminate structure.

In the first waveguide region. C₁, since light is confined by means ofthe actual difference in the refractive index between the active layerand the regions located on both sides thereof by limiting the width ofthe active layer by the first ridge, it is made possible to confinelight more effectively.

Thus it is made possible to surely suppress the unnecessary transversemode of oscillation and control the transverse mode more effectively.

According to the present invention, as described above, by providing thefirst waveguide region C₁ having excellent controllability of transversemode in a part of the waveguide region, unnecessary transverse mode ofoscillation in the first waveguide region C₁ is suppressed therebyimproving the controllability of transverse mode of the entire device,and it is made possible to easily obtain laser devices of various beamcharacteristics.

With the laser device of the present invention, laser beam of a desiredconfiguration can be easily achieved by forming the first waveguideregion C₁ on one end so as to include the resonance end face of thelaser resonator. In other words, it is preferable to form the laserresonance end face 4 so as to correspond to the end face of the firstwaveguide region C₁ as shown in FIG. 3B, FIG. 4A and FIG. 4B. This isbecause, when the region in the vicinity of the resonance end face isturned into the first waveguide region C₁, the transverse mode of lightcan be controlled before and after reflection on the resonance end face,so that the control of the transverse mode functions more effectively inthe waveguide than in a case of providing in other region.

Also according to the present invention, the laser device havingexcellent beam characteristics such as F.F.P. and laser beam aspectratio can be obtained by using the end face of the first waveguideregion C₁ as the laser resonance end face and using the laser resonanceend face as the light emitting plane. This is because, with thisconstitution, by providing the first waveguide region C₁ on the laserbeam emitting plane, it is made easier to control the transverse mode inthe first waveguide region C₁, so that the beam characteristic can beeasily controlled. In case the first waveguide region C₁ is constitutedfrom the first stripe ridge 201 as shown in FIGS. 3, 4, the transversemode can be easily controlled and the desired beam characteristic can beobtained with high accuracy by adjusting the width of stripe of thefirst ridge 201.

Length of the first waveguide region C₁ provided on the light emittingplane may be at least one wavelength of the light emitted by the laser,though a length of several times the wavelength is preferable inconsideration of the function to control the transverse oscillation modein which case desired beam characteristic can be achieved.

Specifically, it is preferable to form the first waveguide region C₁with a length of 1 μm or longer, which enables satisfactory control ofthe transverse oscillation mode. When consideration is given to themanufacturing process, it is preferable to form the first waveguideregion with a length of 5 μm or longer since the stripe ridge 201 can beformed with better accuracy with this length.

Width of the active layer (length in the direction perpendicular to theresonator direction) may be 10 μm, preferably 50 μm or longer and morepreferably 100 μm or longer. In such a constitution as a pair ofpositive and negative electrodes oppose each other via a substrate,width of the active layer becomes equivalent to the chip width. In sucha constitution as a pair of positive and negative electrodes is providedon the same side of a substrate, a surface is exposed to form electrodesin the layer of the first conductivity type thereon, the length is thechip width minus the width of the portion which is removed to form theexposed surface.

[Constitution of Waveguide]

The laser device of the present invention is characterized by the stripewaveguide region having at least the first waveguide region C₁ and thesecond waveguide region C₂, so that the characteristics of the laserdevices can be easily modified by changing the arrangement of thewaveguide regions in the resonator without modification of thecomplicated device design. Specifically, by disposing the firstwaveguide region C₁ on the resonance end face as described above, beamcharacteristic can be easily controlled and desired characteristic canbe easily obtained. Also by setting the proportion of the waveguideoccupied by the first waveguide region C₁ wherein the side face of theactive layer is exposed smaller than that of the second waveguide regionC₂, the laser device of higher reliability can be obtained. This isbecause the proportion of the active layer which is not damaged byetching can be increased by providing more second waveguide region C₂ inthe waveguide. As a result, service life of the device can be elongatedand variations in the service life among the devices can be decreased.

While the laser device of the present invention has at least the firstwaveguide region C₁ and the second waveguide region C₂ as the waveguide,a waveguide region of a configuration other than the first waveguideregion C₁ and the second waveguide region C₂ may also be provided. Forexample, a flat surface 203 formed to incline between the firstwaveguide region C₁ and the second waveguide region C₂ as shown in FIG.4A may be used. Thus in addition to the first waveguide region C₁ andthe second waveguide region C₂, a waveguide different from these may beprovided. Further, the first waveguide region C₁ and the secondwaveguide region C₂ may be provided, one each, in the waveguide or maybe provided in plurality as shown in FIG. 4B. Also nothing may beprovided between the first waveguide region C₁ and the second waveguideregion C₂ as shown in FIG. 3 and FIG. 4B, or an inclination reverse tothat shown in FIG. 4A may be provided so that the first waveguide regionC₁ and the second waveguide region C₂ partially overlap each other.

The laser device of the present invention may also have such aconstitution as a third waveguide region C₃ is formed in addition to thefirst waveguide region C₁ and the second waveguide region C₂ so that theside face of the active layer (side face of waveguide layer) 204 isinclined against the resonator direction. FIG. 13A is a schematicperspective view of the device structure, and FIG. 13B is a sectionalview showing a portion near the junction between the upper claddinglayer 7 and the active layer 3. In this constitution, the thirdwaveguide region C₃ shares the stripe ridge 202 on the upper claddinglayer 7 with the second waveguide region C₂, and the end face (sideface) 204 of the active layer (waveguide layer) is provided in aninclined configuration. In the laser device having the constitutiondescribed above, light guided by the side face 204 can be reflectedcompletely by adjusting the angle α between the resonator direction AAand the direction BB of the active layer side face, as shown in FIG.13B, thus making it possible to guide the light into the first waveguideregion C₁ first waveguide region C₁ striped configuration. Specifically,when the angle α is 70° or less, the incident angle of light in thedirection AA of the resonator on the end face 204 can be set to 20 orgreater so that total reflection without loss can be achieved. Thus theangle α can be set in a range from 0 to 70° according to theapplication. For example, when the angle α is 20° or less, the incidentangle of light in the direction AA of the resonator on the end face 204can be set to 70° or greater, in which case total reflection withoutloss can be achieved. In the second waveguide region C₂, while thestripe waveguide region is formed by making use of the effectivedifference in refractive index in the active layer (waveguide layer),there exists light tat is guided outside of the waveguide region andthis portion of light is reflected on the end face of the secondwaveguide region C₂.

In this case, when the loss in light increases, output power decreasesleading to a deterioration in the current optical output slopeefficiency. When the second waveguide region C₂ is wider than the firstwaveguide region C₁, providing the third waveguide region C₃ between thesecond waveguide region C₂ and the first waveguide region C₁ decreasesthe light loss, thus making it possible to guide the lightsatisfactorily in the junction with the first waveguide region C₁ asshown in FIG. 13.

In the laser device of the present invention, the stripe ridges 201, 202that constitute the first waveguide region C₁ and the second waveguideregion C₂ may have different widths. Beams of different aspect ratioscan be achieved by changing the stripe width. Therefore, the first ridgeand the second ridge can be formed with widths appropriate for theapplication in the laser device of the present invention. While a smallwidth requires an accuracy in the control of the width, it also achievessuch characteristics as FFP near true circle or it is made possible tochange the spread of the beam in correspondence to the width.Specifically, when the width is decreased gradually in a portion 205 ofthe second waveguide region C₂ as shown in FIG. 15, for example, thestripe width in the junction with the first waveguide region C₁ can bemade equal to the stripe width S_(w2), thus making it possible toextract laser beam of various modes in correspondence to the width ofthe first waveguide region C₁. In FIG. 15, a portion where width of thesecond waveguide region C₂ is decreased gradually is shown as the thirdwaveguide region C₃.

In FIG. 15, in order to constitute the second waveguide region C₂ thefirst ridge 202 having width S_(w1) larger than the stripe width S_(w2)of the first ridge that constitutes the first waveguide region C₁ isprovided thereby to form a waveguide which undergoes less variation inthe characteristic with a change in the effective refractive index. Inthe third waveguide region C₃, at the same time, a region 205 havingstripe width inclined in the wave guide is, provided so as to join thewaveguide regions of different stripe widths smoothly, therebyminimizing the loss in the junction. The ridge for constituting thethird waveguide region C₃ may be provided above the active layer asshown in the drawing, or at a depth reaching the layer of firstconductivity by etching similarly to the first waveguide region C₁, orat a position located inbetween.

The stripe ridge for constituting the first and second waveguide regionsof the present invention may be formed in various configurations, forexample in a tapered configuration where the stripe width varies alongthe direction of stripe (longitudinal direction of stripe).Specifically, as exemplified by the first embodiment or shown in FIG.15, in the waveguide structure having the first waveguide region C₁disposed at the light emitting end, the second waveguide region C₂having larger stripe width may be formed in such a configuration thatthe stripe width decreases toward the narrower first waveguide regionC₁, thereby decreasing the light waveguide to the junction of bothportions. Such a tapered stripe may be formed partially as the stripe ofeach waveguide region, or formed in a tapered configuration over theentire length of the stripe, or in such a configuration as a pluralityof tapered stripes having width which decreases toward both endsthereof.

[Stripe in Nitride Semiconductor]

The semiconductor laser device of the present invention constituted fromthe semiconductors of the first conductivity type and the secondconductivity type and the active layer made of nitride semiconductorwill be described below.

The nitride semiconductor used in the laser device of the presentinvention may be GaN, AlN or InN, or a mixed crystal thereof, namely theGroup III-V nitride semiconductor (In_(b)Al_(d)Ga_(1-b-d)N, 0≦b, 0≦d,b+d≦1). Mixed crystals made by using B as the Group III element or bypartially replacing N of the Group V element with As or P may also beused. The nitride semiconductor can be made to have a desiredconductivity type by adding an impurity of appropriate conductivitytype. As an n-type impurity used in the nitride semiconductor, the GroupIV or VI elements such as Si, Ge, Sn, S, O, Ti and Zr may be used, whileSi, Ge or Sn is preferable and most preferably Si is used. As the p-typeimpurity, Be, Zn, Mn, Cr, Mg, Ca or the like may be used, and Mg ispreferably used. As a specific example of the laser device of thepresent invention, a nitride semiconductor laser device will bedescribed below. The nitride semiconductor laser device herein refers toa laser device where nitride semiconductor is used in any of the layerof the first conductivity type, the active layer and the layer of thesecond conductivity type which constitute the laminate, or preferably inall of these layers. For example, cladding layers made of nitridesemiconductor are formed in the layer of the first conductivity type andthe layer of the second conductivity type while the active layer isformed between the two cladding layers thereby forming the waveguide.More specifically, the layer of the first conductivity type includes an-type nitride semiconductor layer and the layer of the secondconductivity type includes a p-type nitride semiconductor layer, whilethe active layer includes nitride semiconductor laser which includes In.

(Active Layer)

According to the present invention, when the semiconductor laser deviceof the present invention is constituted from nitride semiconductor,providing the nitride semiconductor layer which includes In in theactive layer makes it possible to emit laser beam over a range ofwavelengths from blue to red light in the ultraviolet and visibleregions. While the laser device may suffer very serious damage on thenitride semiconductor laser including In when the active layer isexposed to the atmosphere, such a damage to the device can be minimizedaccording to the present invention since the device includes the secondwaveguide region C₂ which is constituted from the first ridge 202provided at such a depth that does not reach the active layer. This isbecause the low melting point of In makes the nitride semiconductorwhich includes In easy to decompose and evaporate and prone to damageduring etching, and makes it difficult to maintain the crystallinityduring the process following the exposure of the active layer, thusresulting in a shorter service life of the device.

FIG. 12 shows the relationship between the depth of etching for formingthe stripe ridge and the device life. As will be seen from the drawing,device life decreases dramatically when etching process reaches theactive layer which has the nitride semiconductor which includes In, andexposure of the active layer leads to serious deterioration of thereliability of the laser device.

Since the laser device of the present invention is provided with thefirst waveguide region C₁ and the second waveguide region C₂ as thewaveguide, the laser device of excellent reliability can be achievedeven in a nitride semiconductor laser device which would otherwiseundergo deterioration in the characteristics when the active layer isexposed to the atmosphere. This is because the first ridge 201 providedfor the constitution of the first waveguide region C₁ constitutes only apart of the waveguide so that reliability of the device can be preventedfrom deteriorating. When length of the resonator is set to about 650 μmand length of the first ridge 201 provided for the constitution of thefirst waveguide region C₁ is set to 10 μm in the nitride semiconductorlaser device of the present invention, for example, it is confirmed thatthe device does not undergo deterioration in reliability due to theactive layer being exposed in the first ridge, and service life ofseveral thousands of hours is ensured with operation of 5 mW in outputpower.

In the nitride semiconductor laser device of the present invention,width of the stripe of the ridge that constitutes the first waveguideregion C₁ or the second waveguide region C₂ is preferably set in a rangefrom 0.5 to 4 μm, or more preferably in a range from 1 to 3 μm in whichcase it is made possible to oscillate in stable transverse mode wit thefundamental (single) mode. When stripe width of the ridge is less than 1μm, it becomes difficult to form the ridge, while width of 3 μm orgreater may cause multi mode oscillation in the transverse modedepending on the wavelength of laser oscillation, and width of 4 μm orgreater may make it impossible to achieve stable transverse mode.According to the present invention controlling the width in a range from1.2 to 2 μm makes it possible to further effectively stabilize thetransverse mode in a high optical output power (effectively suppress theoscillation in unnecessary transverse mode). According to the presentinvention, while it is good for the stripe width of the ridge wheneither of the first waveguide region C₁ or the second waveguide regionC₂ is within the range described above, it is preferable to set thestripe ridge 201 of the first waveguide region C₁ within the rangedescribed above in case the first waveguide region C₁ is provided on thelight emitting side of the resonator plane. Also the present inventionis not limited to such a narrow stripe structure as described above, andmay be applied to a stripe having a width of 5 μm or greater. Also insuch a constitution as the first waveguide region C₁ is disposed on theend of the waveguide, the stripe width of the second waveguide region C₂can be set relatively freely for the control of the laser beamcharacteristic by means of mainly the first waveguide region C₁.

In the nitride semiconductor laser device of the present invention, whenthe end face of the first waveguide region C₁ is used as the resonanceend face (light emitting plane), the laser device having excellentcontrollability of transverse mode, F.F.P. aspect ratio and devicereliability can be obtained. This is because, as described previously,light emitted from the laser device can be controlled immediately beforethe emission by etching deeper than the active layer thereby providingthe first waveguide region C₁ on the light emitting side of theresonator plane, thereby making it possible to obtain laser beams ofvarious shapes and spot sizes.

The active layer may have quantum well structure and, in that case, maybe either single quantum well or multiple, quantum well structure. Highpower laser device and end face light emitting device of excellent lightemitting efficiency can be made by employing the quantum well structure.The second stripe ridge 202 provided for constituting the secondwaveguide region C₂ is formed by etching to such a depth that does notreach the active layer. In this specification, the statement that thesecond stripe ridge 202 is located above the active layer means that theformation by etching to such a depth that does not reach the activelayer. That is, the second stripe ridge 202 that constitutes the secondwaveguide region C₂ is positioned above the interface between the activelayer and the layer formed in contact and above thereof.

The active layer of the nitride semiconductor is preferably the nitridesemiconductor which includes In as described above, and specifically anitride semiconductor represented by Al_(x)In_(y)Ga_(1-x-y)N (0≦x≦1,0<y≦1, x+y≦1) is preferably used. In this case, the nitridesemiconductor described here is preferably used as the well layer in theactive layer of quantum well structure. In the wavelength region (from380 nm to 550 nm) ranging from near ultraviolet to visible green light,In_(y)Ga_(1-y)N (0<y0) is preferably used. Also in a region of longerwavelengths (red), In_(y)Ga_(1-y)N (0<y0) can be used similarly and, atthis time, laser beam of a desired wavelength can be emitted by changingthe proportion y of mixing In. In a region of wavelengths shorter than380 nm, since the wavelength which corresponds to the forbidding bandwidth of GaN is 365 nm, band gap energy nearly equal to or greater thanthat of GaN is required, and therefore Al_(x)In_(y)Ga_(1-x-y)N (0<x≦1,0<y≦1, x+y<1) is used.

In case the active layer is formed in the quantum well structure,thickness of the well layer is in a range from 10 Å to 300 Å, andpreferably in a range from 20 Å to 200 Å, which allows it to decrease Vfand the threshold current density. When the crystal is taken intoconsideration, a layer of relatively homogeneous quality without muchvariations in the thickness can be obtained when the thickness is 20 Åor greater, and the crystal can be grown while minimizing the generationof crystal defects by limiting the thickness within 200 Å. There is nolimitation on the number of well layers in the active layer, which maybe 1 or more. When four or more active layers with larger thickness oflayers constituting the active layer, total thickness of the activelayers becomes too large and the value of Vf increases. Therefore, it isdesirable to restrict the thickness of the well layer within 100 Åhereby to restrain the thickness of the active layer. In the case of LDand LED of high output power, setting the number of well layers in arange from 1 to 3 makes it possible to obtained devices of high lightemission efficiency and is desirable.

The well layer may also be doped or undoped with p- or n-type impurity(acceptor or donor). When nitride semiconductor which includes In isused as the well layer, however, increase in the concentration of n-typeimpurity leads to lower crystallinity and therefore it is preferable torestrict the concentration of n-type impurity thereby to achieve makethe well layer of good crystallinity. Specifically, in order to achievebest crystallinity, the well layer is preferably grown without dopingwith the n-type impurity concentration kept within 5×10¹⁶/cm³. The stateof the n-type impurity concentration kept within 5×10¹⁶/cm³ means anextremely low level of concentration of n-type impurity, and the welllayer can be regarded as including substantially no n-type impurity.When the well layer is doped with n-type impurity, controlling then-type impurity concentration within a range from 1×10¹⁸/cm³ to5×10¹⁶/cm³ makes it possible to suppress the degradation ofcrystallinity and increase the carrier concentration.

There is no limitation to the composition of the barrier layer, andnitride semiconductor similar to that of the well layer can be used.Specifically, a nitride semiconductor which includes In such as InGaNhaving lower proportion of In than the well layer, or a nitridesemiconductor which includes Al such as GaN, AlGaN may be used. Band gapenergy of the barrier layer must be higher than that of the well layer.Specific composition may be In_(β)Ga_(1-β)N (0≦β1, α>β), GaN,Al_(γ)Ga_(1-γ)N (0<γ≦1), and preferably In_(β)Ga_(1-β)N (0≦β<1, α>β),GaN which makes it possible to form the barrier layer of goodcrystallinity. This is because growing a well layer made of a nitridesemiconductor which includes In directly on a nitride semiconductorwhich includes Al such as AlGaN leads to lower crystallinity, eventuallyresulting in impeded function of the well layer. When Al_(γ)Ga_(1-γ)N(0<γ≦1) is used in the barrier layer, the above problem can be avoidedby providing the barrier layer which includes Al on the well layer andproviding a multi-layered barrier layer comprising In_(β)Ga_(1-β)N(0≦β<1, α>β), GaN below the well layer. Thus in the multiple quantumwell structure, the barrier layer sandwiched between the active layersis not limited to a single layer (well layer/barrier layer/well layer),and two or more barrier layers of different compositions and/or impurityconcentrations may be stacked such as well layer/barrier layer(1)/barrier layer (2)/well layer. Letter a represents the proportion ofIn in the well layer, and it is preferable to make the proportion of Inβ in the barrier layer lower than that of the well layer as α>β.

The barrier layer may be doped or undoped with the n-type impurity, butpreferably doped with the n-type impurity. When doped, the n-typeimpurity concentration in the barrier layer is preferably 5×10¹⁶/cm³ orhigher and lower than 1×10²⁰/cm³. In the case of LED which is notrequired to have a high output power, for example, the n-type impurityconcentration is preferably in a range from 5×10¹⁶/cm³ to 2×10¹⁸/cm³.For LED of higher output power and LD, it is preferable to dope in arange from 5×10¹⁷/cm³ to 1×10²⁰/cm³ and more preferably in a range from1×10¹⁸/cm³ to 5×10¹⁹/cm³. When doping to such a high concentration, itis preferable to grow the well layer without doping or withsubstantially no n-type impurity included. The reason for the n-typeimpurity concentration being different among the regular LED, thehigh-power LED and the high-power LD (output power in a range from 5 to100 mW) is that a device of high output power requires higher carrierconcentration in order to drive with larger current for higher outputpower. Doping in the range described above, as described above, it ismade possible to inject the carrier to a high concentration with goodcrystallinity.

In the case of a nitride semiconductor device such as lower-power LD,LED or the like, in contrast, a part of the barrier layer of the activelayer may be doped with the n-type impurity or the entire barrier layersmay be formed with substantially no n-type impurity included. Whendoping with the n-type impurity, all the barrier layers of the activelayer may be doped or a part of the barrier layers may be doped. Whenpart of the barrier layers is doped with the n-type impurity, it ispreferable to dope the barrier layer which is disposed on the n-typelayer side in the active layer. Specifically, when the barrier layer Bn(n=1, 2, 3 . . . ) which is nth layer from the n-type layer side,electrons are effectively injected into the active layer and a devicehaving excellent light emission efficiency and quantum efficiency can bemade. This also applies to the well layer, as well as the barrier layer.When both the barrier layer and the active layer are doped, the effectdescribed above can be achieved by doping the barrier layer Bn (n=1, 2,3 . . . ) which is nth layer from the n-type layer side and the mth welllayer Wm (m=1, 2, 3 . . . ), namely doping the layer nearer to then-type layer first.

While there is no limitation to the thickness of the barrier layer, thethickness is preferably not larger than 500 Å, and more specificallyfrom 10 to 300 Å similarly to the active layer.

In the nitride semiconductor laser device of the present invention, itis preferable that the laminate structure includes the n-type nitridesemiconductor layer for the layer of first conductivity type and thep-type nitride semiconductor for the layer of second conductivity type.Specifically, the n-type cladding layer and the p-type cladding layerare provided in the layers of the respective types, thereby to form thewaveguide. At this time, a guide layer and/or an electron confinementlayer may be formed between the cladding layers and the active layer.

(p-Type Cladding Layer)

In the nitride semiconductor laser device of the present invention, itis preferable to provide the p-type cladding layer which includes thep-type nitride semiconductor (first p-type nitride semiconductor) as thelayer of second conductivity type or the layer of first conductivitytype. In this case, the waveguide is formed in the laminate structure byproviding the n-type cladding layer which includes the n-type nitridesemiconductor layer in the layer of the conductivity type different fromthat of the layer wherein the p-type cladding layer is provided. Thenitride semiconductor used in the p-type cladding layer is required onlyto have a difference in the refractive index large enough to confinelight, and nitride semiconductor layer which includes Al is preferablyused. This layer may be either a single layer or a multi-layered film.Specifically, a super lattice structure having AlGaN and GaN stacked oneon another achieves better crystallinity and is therefore preferable.This layer may be either doped with p-type impurity or not doped. For alaser device oscillating at a long wavelength in a range from 430 to 550nm, the cladding layer is preferably made of GaN doped with p-typeimpurity. While there is no limitation to the film thickness, thicknessin a range from 100 Å to 2 μm, or more preferably from 500 Å to 1 μmmakes the film function satisfactorily as the light confinement layer.

Also according to the present invention, the electron confinement layerand/or the optical guide layer to be described later may be providedbetween the active layer and the p-type cladding layer. When providingthe optical guide layer, it is preferable provide the optical guidelayer also between the n-type cladding layer and the active layer, insuch a structure as the active layer is sandwiched by optical guidelayers. This creates SCH structure in which light can be confined by thecladding layer by making the proportion of Al content higher in thecladding layer than in the guide layer thereby providing a difference inrefractive index. In case the cladding layer and the guide layer areformed in multi-layered structure, proportion of Al content isdetermined by the mean proportion of Al.

(p-Type Electron Confinement Layer)

The p-type electron confinement layer which is provided between theactive layer and the p-type cladding layer, or preferably between theactive layer and the p-type optical guide layer also function to confinethe carrier in the active layer thus making it easier to oscillate byreducing the threshold current, and is made of AlGaN. Particularly moreeffective electron confinement can be achieved by providing the p-typecladding layer and the p-type electron confinement layer in the layer ofsecond conductivity type. When AlGaN is used for the p-type electronconfinement layer, while the above mentioned function can be reliablyachieved by doping with the p-type impurity, carrier confining functioncan also be achieved even without doping. Minimum film thickness is 10 Åand preferably 20 Å. The above mentioned function will be achievedsatisfactorily by forming the film to a thickness within 500 Å andsetting the value of x in formula Al_(X)Ga_(1-X)N to 0 or larger,preferably 0.2 or larger. The n-type carrier confinement layer may alsobe provided on the m-type layer side for confining the holes within theactive layer. Confinement of holes can be done without making such anoffset (difference in the band gap from the active layer) as in the caseof electron confinement. Specifically, a composition similar to that ofthe p-type electron confinement layer may be used. In order to achievegood crystallinity, this layer may be formed from a nitridesemiconductor layer which does not includes Al, and a compositionsimilar to that of the barrier layer of the active layer may be used. Inthis case, it is preferable to dispose the n-type barrier layer whichconfines the carrier nearest to the n-type layer in the active layer, orwithin the n-type layer in contact with the active layer. Thus byproviding the p-type and n-type carrier confinement layers in contactwith the active layer, the carrier can be injected effectively into theactive layer or into the well layer. In another form, a layer whichmakes contact with the p-type or n-type layer in the active layer can beused as the carrier confinement layer.

[p-Type Guide Layer]

According to the present invention, good waveguide can be formed fromnitride semiconductor by providing the guide layer which sandwiches theactive layer at a position inside of the cladding layer thereby formingthe optical waveguide. In this case, thickness of the waveguide (theactive layer and the guide layers on both sides thereof) is set towithin 6000 Å for suppressing abrupt increase in the oscillationthreshold current. Preferably the thickness is within 4500 Å to makecontinuous oscillation possible with long service life at a restrictedthreshold current in the fundamental mode. Both guide layers arepreferably formed to substantially the same thickness in a range from100 Å to 1 μm, more preferably in a range from 500 Å to 2000 Å in orderto form good optical waveguide. The guide layer is made of nitridesemiconductor, while it suffices to have a band gap energy sufficient toform the waveguide compared to the cladding layer to be provided on theoutside thereof, and may be either a single film or a multi-layeredfilm. Good waveguide can be formed by making the optical guide layerhaving a band gap energy equal to or greater than that of the activelayer. In the case of quantum well structure, band gap energy is madegreater than that of the well layer, and preferably greater than that ofthe barrier layer. Further, good optical waveguide can be formed byproviding a band gap energy for about 10 nm or larger than thewavelength of light emitted in the active layer in the optical guidelayer.

For the p-type guide layer, it is preferable to use undoped GaN in therange of oscillation wavelengths from 370 to 470 nm, and use amulti-layered structure of InGaN/GaN in a range of relatively longwavelengths (450 μm and over). This makes it possible to increase therefractive index in the waveguide constituted from the active layer andthe optical guide layer, thereby increasing the difference in therefractive index from the cladding layer. In a range of shorterwavelengths within 370 nm, nitride semiconductor which includes Al ispreferably used since the absorption, edge is at 365 nm. Specifically,Al_(x)Ga_(1-x)N (0<x<1) is preferably used to form a multi-layered filmmade of AlGaN/GaN, multi-layered film made by alternate stacking thereofor a super lattice multi-layered film in which each layer has superlattice structure. Constitution of the n-type guide layer is similar tothat of the p-type guide layer. Satisfactory waveguide can be can bemade by using GaN, InGaN in consideration of the energy band gap of theactive layer, and forming multi-layered film comprising InGaN and GaNstacked alternately with the proportion of In content being decreasedtoward the active layer.

(n-Type Cladding Layer)

In the nitride semiconductor laser device of the present invention,nitride semiconductor used in the n-type cladding layer is required onlyto have a difference in the refractive index large enough to confinelight similarly to the p-type cladding layer, and a nitridesemiconductor layer which includes Al is preferably used. This layer maybe either a single layer or a multi-layered film. Specifically, a superlattice structure having AlGaN and GaN stacked one on another. Then-type cladding layer functions as the carrier confinement layer and thelight confinement layer. In case multi-layered structure is employed, itis preferable to grow nitride semiconductor layer including Al,specifically AlGaN as described previously. Further, this layer may beeither doped with n-type impurity or not doped, and also one of theconstituting layers may be doped. For a laser device oscillating at along wavelength in a range from 430 to 550 nm, the cladding layer ispreferably made of GaN doped with n-type impurity. While there is nolimitation to the film thickness, similarly to the case of the p-typecladding layer, thickness in a range from 100 Å to 2 μm, or morepreferably from 500 Å to 1 μm makes the film function satisfactorily asthe light confinement layer.

In the nitride semiconductor laser device, good insulation can beachieved by locating the position, where the stripe ridge is formed,within the nitride semiconductor layer which includes Al and providingan insulation film on the exposed nitride semiconductor surface and onthe side face of the ridge. A laser device without leak current can alsobe made by providing electrodes on the insulation film. This is becausealmost no material exists that can achieve good ohmic contact in thenitride semiconductor layer which includes Al, and good insulationwithout leak current can be achieved by forming the insulation film andelectrode on the semiconductor surface. When the electrode is providedon the nitride semiconductor layer which does not include Al, incontrast, ohmic contact can be easily formed between the electrode andthe nitride semiconductor. When the electrode is formed on the nitridesemiconductor layer which does not include Al via the insulation film,microscopic pores in the insulation film cause leakage depending on thefilm quality of the insulation film and the electrode. In order to solvethis problem, it is necessary to form the insulation film having athickness sufficient to provide the required level of insulation or todesign the shape and position of the electrode so as not to overlap thesemiconductor surface, thus imposing a significant restraint on thedesign of the laser device constitution. It is important where toprovide the ridge, because the surface of the nitride semiconductor onboth sides of the ridge exposed when forming the ridge has far greaterarea than the side face of the ridge, and satisfactory insulation can besecured in this surface. Thus a laser device having a high degree offreedom in the design can be made where electrodes of variousconfigurations can be applied and the location of forming the electrodecan be determined relatively freely, which is very advantageous informing the ridge. For the nitride semiconductor layer which includesAl, AlGaN or the super lattice multi-layered structure of AlGaN/GaNdescribed above is preferably used.

The first ridge 201 and the second stripe ridge 202 of stripedconfiguration provided as the first waveguide region C₁ and the secondwaveguide region C₂ are formed by removing both sides of each ridge asshown in FIGS. 1B and 1C. The ridge 202 is provided on the uppercladding layer 7 and the surface of the upper cladding layer 7 exposedin a region other than the ridge determines the depth of etching.

[Electrode]

The laser device of the present invention is not limited to theelectrode configuration provided on the stripe ridge and the secondridge. As shown in FIG. 1 and FIG. 7, for example, the electrode may beformed on almost the entire surface of the first stripe ridge 201 andthe second stripe ridge 202 provided as the first waveguide region C₁and the second waveguide region C₂. Also the electrode may be providedon the second waveguide region C₂ only thereby injecting the carrierinto the second waveguide region C₂ with preference. On the contrary,the electrode may be provided on the first waveguide region, C₁ only,with the waveguide being functionally separated in the direction ofresonator.

[Insulation Film]

In the laser device of the present invention, in case a part of thelaminate is removed and a stripe ridge is provided to form theresonator, it is preferable to form the insulation film on the side faceof the stripe and on the plane (surface whereon the ridge is provided)on both sides of the ridge which continues thereto. For example, afterthe stripe ridge shown in FIG. 1 is provided, the insulation film isprovided in such a way as to extend from the side face of the ridge tothe surfaces on both sides of the ridge.

In case nitride semiconductor is used in the laser device of the presentinvention, it is preferable to provide a second protective film 162 asan insulation film as shown in FIGS. 7, 8, 9.

For the second protective film, a material other than SiO₂, preferablyan oxide which includes at least one kind of element selected from amongthe group consisting of Ti, V, Zr, Nb, Hf and Ta, or at least one ofSiN, BN, SiC and AlN is used and, among these, it is particularlypreferable to use Zr or Hf, or BN, SiC. While some of these materialsare slightly soluble to hydrofluoric acid, use of these materials as theinsulation layer of the laser device will achieve reliability fairlyhigher than SiO₂ as a buried layer. In the case of a thin film made ofan oxide which is formed in vapor phase such as PVD or CVD, since it isgenerally difficult for the element and oxygen to reactstoichiometrically to form the oxide, reliability tends to be lower forthe insulation of the thin film of oxide. In contrast, oxides of theelement selected in the present invention formed by PVD or CVD, and BN,SiC or AlN have higher reliability of insulation property than Si,oxide. Moreover, when an oxide having a refractive index lower than thatof the nitride semiconductor (for example, one other than SiC) isselected, a buried layer of laser device can be favorably formed.Further, when the first protective film 161 is formed from Si oxide,since the Si oxide can be, removed using hydrofluoric acid, the secondprotective film 162 having uniform thickness can be formed on thesurface except for the top surface of the ridge as shown in FIG. 7C, byforming the first protective film 161 only on the top surface of theridge as shown in FIG. 7B, forming the second protective film 162continuously on the first protective film 161, the side faces of theridge and the surfaces on both sides of the ridge (etching stopperlayer), and selectively removing the first protective film 161.

Thickness of the second protective film is in a range from 500 Å to 1μm, and preferably in a range from 1000 Å to 5000 Å. When the thicknessis less than 500 Å, sufficient insulation cannot be achieved whenforming the electrode. When thicker than 1 μm, uniformity of theprotective film cannot be achieved and good insulation film cannot beobtained. When the thickness is in the preferred range described above,a uniform film having a favorable difference in refractive index fromthat of the ridge can be formed on the side face of the ridge.

The second protective film can also be formed by means of buried layerof nitride semiconductor. In the case of semi-insulating, i-type nitridesemiconductor, type of conductivity opposite to that of the ridge of thewaveguide region, for example in the second waveguide region C₂ of thefirst embodiment, a buried layer made of n-type nitride semiconductorcan be used as the second protective film. As a specific example ofburied layer, confinement of light in the transverse direction can beachieved by providing a difference in refractive index from the ridge bymeans of a nitride semiconductor, layer which includes Al such as AlGaNor achieving the function of current blocking layer, and good opticalproperty of the laser device can be achieved by providing a differencein the light absorption coefficient by means of a nitride semiconductorlaser which includes In. When a layer other than semi-insulating, i-typelayer is used for the buried layer, the second waveguide region may be aburied layer of the first conductivity type different from the secondconductivity type. In the first ridge that constitutes the firstwaveguide region, on the other hand, since the layers of the first andsecond conductivity types are formed in stripe configuration on bothsides of the active layer, a buried layer of the second conductivitytype different from the first conductivity type is formed in the layerof the first conductivity type or in the regions on both sides of thelayer of the first conductivity type and the active layer, while aburied layer of the first conductivity type different from the secondconductivity type is formed in the layer of the second conductivity typeor in the regions on both sides of the layer of the second conductivitytype and the active layer. As described above, the buried layer may beformed in different constitutions in the first waveguide region and thesecond waveguide region. The buried layer is formed on a part of thestripe side face, or preferably over substantially the entire surface,similarly to the second protective film. Moreover, when the buried layeris formed on the side face of the ridge and the surface of the nitridesemiconductor on both sides of the ridge, better light confinementeffect and current pinching effect can be achieved. Such a constitutionmay also be employed as, after forming the buried layer, a layer ofnitride semiconductor is formed on the buried layer and/or the stripeand ridges constituting the waveguide regions are disposed in thedevice.

Length of the resonator of the nitride semiconductor laser device of thepresent invention may be in a range from 400 to 900 μm, in which casethe drive current can be decreased by controlling the reflectance of themirrors on both ends.

[Manufacturing Method]

As described above, the nitride semiconductor laser device of thepresent invention can achieve good device characteristics. Further, thestripe waveguide region of the laser device of the present invention canbe made with a high accuracy and high yield of production, by formingthe stripes that make the first waveguide region C₁ and the secondwaveguide region C₂ in the process described below. The manufacturingmethod also makes it possible to manufacture the laser device havinghigh reliability. The manufacturing method will now be described indetail below.

As shown in FIGS. 8 and 9, when manufacturing a device having a pair ofpositive and negative electrodes formed on the same side of differentkind of substrate, in order to expose an n-type contact layer whereonthe negative electrode is to be formed as shown in FIG. 7, etching isdone to that depth followed by etching to form the stripe waveguideregion.

(Method 1 for Forming the Stripe Ridge)

FIG. 5 is a schematic perspective view showing a part of a wafer whereondevice structure is formed from nitride semiconductor, for explainingthe process of forming the electrodes according to the presentinvention. FIG. 6 is a similar drawing for explaining another embodimentof the present invention. FIG. 7 shows a process after forming thesecond protective film, FIG. 7B showing sectional view of the secondwaveguide region C₂ in FIG. 7A and FIG. 7C showing sectional view of thesecond waveguide region C₂ in FIG. 7D. According to the manufacturingmethod of the present invention, as shown in FIG. 5A, after stacking thesemiconductor layers that constitute the device structure, the firstprotective film 161 of stripe configuration is formed on a contact layer8 in the layer of second conductivity type on the top layer.

The first protective film 161 may be made of any material as long as ithas a difference from the etching rate of the nitride semiconductor,whether insulating or not. For example, Si oxide (including SiO₂),photoresist or the like is used, and such a material that is moresoluble to an acid than the second protective film does is preferablyused in order to differentiate the solubility against the secondprotective film which will be formed later. Hydrofluoric acid ispreferably used for the acid, and accordingly Si oxide is preferablyused as the material soluble to hydrofluoric acid. Stripe width (W) ofthe first protective film is controlled within a range from 1 μm to 3μm. Stripe width of the first protective film 161 roughly corresponds tothe stripe width of the ridge that constitutes the waveguide region.

FIG. 5A shows the first protective film 161 being formed on the surfaceof the laminate. That is, the first protective film 161 having such astripe configuration as shown in FIG. 5A is formed on the surface of thecontact layer 8 by, after forming the second protective film oversubstantially the entire surface of the laminate, forming a mask of adesired shape on the surface of the first protective film byphotolithography process.

Lift-off method may also be employed to form the first protective film161 having such a stripe configuration as shown in FIG. 5A. That is,after forming a photoresist having slits formed in stripe configuration,the first protective film is formed over the entire surface of thephotoresist, and the photoresist is removed by dissolving therebyleaving only the first protective film 161 which is in contact with thecontact layer 8. Well-shaped, stripes having substantially vertical endfaces can be obtained by the etching process described above rather thanby forming the first protective film of stripe configuration by thelift-off method.

Then as shown in FIG. 5B, the first protective film 161 is used as themask for etching from the contact layer 8 the portion where the firstprotective film 161 is not formed, thereby to form the stripe ridgeaccording to the shape of the protective film directly below the firstprotective film 161. When etching, structure and characteristic of thelaser device vary depending on the position to stop etching.

As the means for etching the layer formed from nitride semiconductor,dry etching is used such as RIE (reactive ion etching). For etching thefirst protective film made of Si oxide, it is preferable to use gas offluorine compound such as CF₄. For etching the nitride semiconductor inthe second process, use of gas of chlorine compound such as Cl₂, CCl₄and SiCl₄ which are commonly used for the other Group III-V compoundsemiconductor makes the selectivity with respect to the Si oxide higher,and is therefore desirable.

Then as shown in FIG. 5C, a third protective film 163 is formed so as tocover a part of the stripe ridge. For the third protective film 163,known resist film which has resistance to dry etching can be used, suchas light hardening resin. At this time, the stripe ridge covered by thethird protective film 163 becomes the second ridge 202 for constitutingthe second waveguide region C₂, and the first ridge 201 whichconstitutes the first waveguide region C₁ is formed in a region notcovered by third protective film. The third protective film 163 and thefirst protective film 161 formed as described above are used to etch thelaminate, where the masks are not formed, to such a depth as to reachthe cladding layer, thereby to form the stripe ridges (first ridge) ofdifferent depths.

Then as shown in FIG. 7A, the second protective film 162 of aninsulating material different from that of the first protective film 161is formed on the side faces of the stripe ridge and on the surfaces ofthe layers which have been exposed by etching (cladding layers 5, 7 inFIG. 7). The first protective film 161 is made of a material differentfrom that of the second protective film 162, so that the firstprotective film 161 and the second protective film 162 are selectivelyetching. As a result, when only the first protective film 161 is removedby, for example, hydrofluoric acid, the second protective film 162 canbe formed continuously over the surfaces of the cladding layers 5, 7(the surfaces of the nitride semiconductor which have been exposed byetching) and the side faces of the ridge with the top surface of theridge being opened as shown in FIG. 7B. By forming the second protectivefilm 162 continuously as described above, high insulation property canbe maintained. In addition, when, the second protective film 162 isformed continuously over the first protective film 161, the film can beformed with uniform thickness on the cladding layers 5, 7, and thereforecurrent concentration due to uneven film thickness does not occur. Sincethe etching is stopped amid the cladding layers 5, 7, the secondprotective film 162 is formed below the surfaces of the cladding layers5, 7 (top surfaces which are exposed). However, the second protectivefilm is formed on the layer where the etching was stopped when theetching is stopped below the cladding layers 5, 7, as a matter of fact.

In the next process, the first protective film 161 is removed bylift-off as shown in FIG. 7B. Then the electrode is formed on the secondprotective film 162 and the contact layer 8 so as to electricallycontact the contact layer 8. According to the present invention, sincethe second protective film having the striped openings is formed firston the ridge, it is not necessary to form the electrode only on thecontact layer of narrow stripe width, and it is made possible to formthe electrode of a large area which continues from the contact layerthat is exposed through the opening to the second insulation film. Thismakes it possible to form the electrode combining the electrode forohmic contact and the electrode for bonding together, by selecting theelectrode material that combines the function of ohmic contact.

When forming the stripe waveguide region in the nitride semiconductorlaser device, dry etching is employed because it is difficult to etch bythe wet process. Since selectivity between the first protective film andthe nitride semiconductor is important in the dry etching process, SiO₂is used for the first protective film. However, sufficient insulationcannot be achieved when SiO₂ is used also in the second protective filmformed on the top surface of the layer where etching has been stopped,and the material is the same as that of the first protective film, itbecomes difficult to remove the protective film only. For this reason, amaterial other than SiO₂ is used for the second protective film therebyensuring the selectivity with respect to the first protective film inthe present invention. Also because the nitride semiconductor is notetched after forming the second protective film, difference in theetching rate between the second protective film and the nitridesemiconductor makes no problem.

(Method 2 for Forming the Stripe Ridge)

FIG. 16 is a schematic perspective view showing a part of a waferwhereon device structure is formed from nitride semiconductor, forexplaining the process of forming the semiconductor laser according to,the present invention. Processes of this method are substantiallysimilar to the processes of the method 1, although the end faces of theresonator are formed at the same time the n-type contact layer isexposed for forming the negative electrode by etching in the case ofthis method. Namely, the order of forming different portions isdifferent from the method 1. In the method 2, first the n-type contactlayer is exposed (FIG. 16A). At this time, the end faces of theresonator are formed at the same time. Then the stripe ridge, the firstand second waveguide regions and the electrode are formed similarly tothe method 1 (FIG. 16B). By forming the end faces of the resonator byetching first as described above, the invention can also be applied tosuch a case as good end faces of the resonator cannot be obtained bycleaving.

In the laser device of the present invention, as described above, thestripe ridge 202 for constituting the first waveguide region C₁ and thesecond waveguide region C₂ can be efficiently formed, and the electrodecan be formed on the surface of the ridge of the laminate.

(Etching Means)

According to the manufacturing method of the present invention, when dryetching is used such as RIE (reactive ion etching) as the means foretching the layer formed from nitride semiconductor, it is preferable touse gas of fluorine compound such as CF₄ for etching the firstprotective film made of Si oxide which is frequently used in the firstprocess. For etching the nitride semiconductor in the second process,use of gas of chlorine compound such as Cl₂, CCl₄ and SiC ₄ which arecommonly used for the other Group III-V compound semiconductor makes theselectivity with respect to the Si oxide higher, and is thereforedesirable.

(Chip Formation)

FIG. 17 is a schematic sectional view showing the cutting position whenmaking chips out of the laminate formed on the wafer as describedpreviously. FIG. 17A shows only the substrate, and FIG. 17B shows a caseof dividing the substrate and the n-type layer. Regions each including apair of electrodes formed therein are dealt with as units and arereferred to as I, II, III and IV from left to right as shown in thedrawing. Ia, IIa and IVa in FIG. 17A are arranged so that the firstwaveguide region is directed to the right, and IIIa is directedopposite. Ib, IIb and IIIIb in FIG. 17B are arranged so that the firstwaveguide region is directed to the right, and IVb is directed opposite.Such an arrangement of the units before division may be selected asrequired.

When divided along line A-A, end faces of the resonator can be left asformed by etching. In units I and II, an end face on the light reflectorside of the resonator is the cleaved facet when divided along line B-Bafter being divided along line A-A. In II, the end face on the lightemitting side of the resonator is also the cleaved facet when dividedalong line D-D. When divided along line C-C, end faces on the lightreflector side of the resonator in IIIa and IVa are formed as cleavedfacets at the same time. Similarly, when divided along line E-E, endfaces on the light emitting side of the resonator in IIIb and IVb areformed as cleaved facets at the same time. Thus the end face of thedevice and resonator end faces can be formed as etched surface orcleaved surface depending on the cut-off position.

In order to achieve such an arrangement as only the substrate existsbetween the resonator end face of Ia and the resonator end face of IIbas between Ia and IIa shown in FIG. 17A, the work whereon the resonatorend faces have been formed by etching as shown in FIG. 16B is furtheretched down to the substrate. The reason for etching down to thesubstrate is to prevent the semiconductor layer from cracking whendividing. In case the substrate is exposed in a single etching processby skipping the step shown in FIG. 16A, the surface near the activelayer which has been exposed by etching earlier becomes roughened due tolong duration of etching, thus making it difficult to obtain goodresonator end face. When the etching process is divided into two steps,first etching to the n-type layer as shown in FIG. 16A and then etchingdown to the substrate, good resonator end faces can be formed anddivision becomes easier. FIG. 16D shows the work shown in FIG. 16C beingcut off at the position indicated by the arrow. By applying etching intwo steps as described above, protrusion such as D in the drawing isformed. When etching down to the substrate, it is necessary to reducethe length of this protrusion D to the light emitting direction. This isbecause a large width D (length of protrusion) blocks the light emittedfrom the light emitting face and makes it difficult to obtain goodF.F.P. There will be no problem when D is small at least at the end faceon the light emitting side.

(Reflector Film)

FIG. 18 schematically shows the method of forming a reflector film onthe resonator end face. By disposing semiconductor which is divided intobar shape so that the end face on the light reflecting side or the endface on the light emitting side opposes the material of the reflectorfilm, as shown in FIG. 18, the reflector film is formed by sputtering orthe like. By forming the reflector film by sputtering while dividing thesemiconductor into bar shape and disposing the cut-off face to opposethe material of the reflector film, high-quality reflector film whichhas uniform thickness and is less likely to deteriorate can be formedeven when the film is formed in multi-layered structure. Such areflector film is more effective when used in a device which is requiredto have a high output power and, particularly when formed inmulti-layered structure, the reflector film bearable to high outputpower can be made. The reflector film can be formed so as to extend tothe resonator end face which is a side face even by sputtering fromabove the electrode. In this case, however, such an advantage as theprocess to form into bar shape and directing the end face upward can beeliminated, although uniform film cannot be obtained particularly in thecase of in multi-layered film since the film is formed from sidewaysonto the end face and therefore somewhat lower film quality results.Such a reflector film may be provided on both the light reflecting endface and the light emitting end face, or only on one end face anddifferent materials may be used.

According to the present invention, there is no limitation to the otherdevice structure such as the active layer and the cladding layer, andvarious layer structures can be used. As a specific device structure,for example, the device structure shown in the embodiment to bedescribed later may be used. Also there is no limitation to theelectrode, and various constitutions of electrode can be used.Composition of the nitride semiconductors used in various layers of thelaser device is not restricted and nitride semiconductors represented bythe formula In_(b)Al_(c)Ga_(1-b-c)N (0≦b, 0≦d, b+d<1) can be used.

According to the present invention, any known methods of growing nitridesemiconductor such as MOVPE, MOCVD (metalorganic chemical vapor phasedeposition), HVPE (hallide vapor phase epitaxy) and MBE (molecular beamepitaxy).

EMBODIMENTS

Now embodiment of the present invention will be described below.

While the following embodiments deal with laser devices made of nitridesemiconductor, the laser device of the present invention is not limitedto this constitution and the technology of the present invention can beapplied to various semiconductors.

Embodiment 1

A laser device of the first embodiment will be described below.Specifically, the laser device comprising the second waveguide region C₂which has the sectional structure shown in FIG. 8 and the firstwaveguide region C₁ which has the sectional structure shown in FIG. 9 ismade as the first embodiment.

While a substrate made of sapphire, namely a material different from thenitride semiconductor is used in the first embodiment, a substrate madeof nitride semiconductor such as GaN substrate may also be used. As thesubstrate of different material, an insulating substrate such assapphire and spinel (MgAl₂O₄) each having the principal plane in eitherthe C plane, R plane or A plane, SiC (including 6H, 4H and 3C), ZnS,ZnO, GaAs, Si, or an oxide which can be lattice matched with the nitridesemiconductor can be used as long as the nitride semiconductor can begrown on the substrate. As the substrate of different material, sapphireand spinel are preferably used. The substrate of different material mayhave a plane inclined from the low index plane which is commonly used(off-angle), in which case the base layer made of gallium nitride can begrown with good crystallinity by using a substrate which has stepwiseoff-angle configuration.

Also when the substrate of different material is used, after growing thebase layer made of the nitride semiconductor on the substrate, thesubstrate of different material is removed by polishing or other processto leave only the base layer before forming the device structure, andthen the device structure may be formed by using the base layer as thesingle substrate of the nitride semiconductor, or the substrate ofdifferent material may also be removed after forming the devicestructure.

In case the substrate of different material is used as shown in FIG. 8,device structure made of good nitride semiconductor can be formed byforming the device structure after forming the buffer layer and the baselayer thereon. FIG. 8 is a sectional view showing the device structurein the second waveguide region C₂, and FIG. 9 is a sectional viewshowing the device structure in the first waveguide region C₁.

(Buffer Layer 102)

In the first embodiment, first, a substrate 101 of different materialmade of sapphire with the principal plane lying in the C plane havingdiameter of 2 inches is set in a MOVPE reaction vessel, temperature isset to 500° C., and a buffer layer made of GaN is formed to a thicknessof 200 Å by using trimethyl gallium (TMG) and ammonia (NH₃).

(Base Layer 103)

After growing the buffer layer 102, temperature is set to 1050° C. and anitride semiconductor layer 103 made of undoped GaN is grown to athickness of 4 μm by using TMG and ammonia. This layer is formed as thebase layer (substrate for film growth) for the constitution of thedevice. The base layer may also be formed from nitride semiconductor byELOG (Epitaxially Laterally Overgrowth), which makes it possible to growthe nitride semiconductor with good crystallinity. ELOG referscollectively to growing methods accompanied by lateral growth in which,for example, after growing a nitride semiconductor layer on a substrateof different material, the surface is covered by a protective film onwhich it is difficult to grow the nitride semiconductor formed thereonin the configuration of stripes at constant intervals, and nitridesemiconductor is grown newly from the nitride semiconductor surfaceexposed through the slits of the protective film, thereby covering theentire substrate with the nitride semiconductor. That is, when a maskedregion where a mask is formed and a non-masked region where the nitridesemiconductor is exposed are formed alternately and nitridesemiconductor is grown again from the surface of the nitridesemiconductor exposed through the non-masked region, the layer growsfirst in the direction of thickness but eventually grows also in thelateral direction as the growth proceeds so as to cover the maskedregion, thereby to cover the entire substrate.

The ELOG growth processes also include such a process as an opening isformed through which the substrate surface is exposed in the nitridesemiconductor layer which has been grown first on the substrate ofdifferent material, and nitride semiconductor is grown from the nitridesemiconductor located at the side face of the opening sideways, therebyforming the film.

According to the present invention, these various variations of the ELOGgrowth method can be employed. When nitride semiconductor is grown byusing the ELOG growth method, the nitride semiconductor formed by thelateral growth has good crystallinity and therefore a nitridesemiconductor layer having good overall crystallinity can be obtained.

Then the following layers which constitute the device structure arestacked on the base layer made of nitride semiconductor.

(n-Type Contact Layer 104)

First, an n-type contact layer 3 made of GaN doped with Si concentrationof 1×10¹⁸/cm³ is formed to a thickness of 4.5 μm at a temperature of1050° C. on the nitride semiconductor substrate (base layer) 103 byusing TMG, ammonia, and silane gas used as an impurity gas.

(Crack Preventing Layer 105)

Then a crack preventing layer 105 made of In_(0.06)Ga_(0.94)N is formedto a thickness of 0.15 μm at a temperature of 800° C. by using TMG, TMI(trimethyl indium), and ammonia. The crack preventing layer may beomitted.

(n-Type Cladding Layer 106)

After growing layer A made of undoped AlGaN to a thickness of 25 Å isgrown at a temperature of 1050° C. by using TMA (trimethyl aluminum),TMG and ammonia as the stock material gas, supply of TMA is stopped andsilane gas is used as the impurity gas, and layer B made of GaN dopedwith Si concentration of 5×10¹⁸/cm³ is formed to a thickness of 25 Å.This operation is repeated 160 times to stack the layer A and layer B toform the n-type cladding layer 106 made in multi-layered film (superlattice structure) having a total thickness of 8000 Å. At this time, adifference in the refractive index sufficient for the cladding layer tofunction can be provided when the proportion of Al of the undoped AlGaNis in a range from 0.05 to 0.3.

(n-Type Optical Guide Layer 107)

Then at a similar temperature, an n-type optical guide layer 107 made ofundoped GaN is formed to a thickness of 0.1 μm by using TMG and ammoniaas the stock material gas. The n-type optical guide layer 107 may bedoped with an n-type impurity.

(Active Layer 108)

Then by setting the temperature to 800° C., a barrier layer made ofIn_(0.05)Ga_(0.95)N doped with Si in a concentration of 5×10¹⁸/cm³ to athickness of 100 Å by using TMI (trimethyl indium), TMG and ammonia asthe stock material gas and silane gas as the impurity gas. Then thesupply of silane gas is stopped and a well layer made of undopedIn_(0.1)Ga_(0.9)N is formed to a thickness of 50 Å. This operation isrepeated three times thereby to form the active layer 108 of multiplequantum well structure (MQW) having total thickness of 550 Å with thelast layer being the barrier layer.

(p-Type Electron Confinement Layer 109)

Then at a similar temperature, a p-type electron confinement layer 109made of AlGaN doped with Mg in a concentration of 1×10¹⁹/cm³ is formedto a thickness of 100 Å by using TMA, TMG and ammonia as the stockmaterial gas and Cp₂Mg (cyclopentadienyl magnesium) as the impurity gas.This layer may not be provided, though would function as electronconfinement layer and help decrease the threshold when provided.

(p-Type Optical Guide Layer 110)

Then by setting the temperature to 1050° C., a p-type optical guidelayer 110 made of undoped GaN is formed to a thickness of 750 Å by usingTMG and ammonia as the stock material gas.

While the p-type optical guide layer 110 is grown as an undoped layer,diffusion of Mg from the p-type electron confinement layer 109 increasesthe Mg concentration to 5×10¹⁶/cm³ and turns the layer p-type.Alternatively, this layer may be intentionally doped with Mg whilegrowing.

(p-Type Cladding Layer 111)

Then a layer of undoped Al_(0.16)Ga_(0.84)N is formed to a thickness of25 Å at 1050° C., then supply of TMA is stopped and a layer of Mg-dopedGaN is formed to a thickness of 25 Å by using Cp₂. This operation isrepeated to form the p-type cladding layer 111 constituted from superlattice structure of total thickness of 0.6 μm. When the p-type claddinglayer is formed in super lattice structure consisting of nitridesemiconductor layers of different band gap energy with at least onethereof including Al being stacked one on another, crystallinity tendsto be improved by doping one of the layers more heavily than the other,in the so-called modulated doping. In the present invention, however,both layers may be doped similarly. The cladding layer is made in superlattice structure consisting of nitride semiconductor layers whichinclude Al, preferably Al_(X)Ga_(1-X)N (0<X<1), more preferably superlattice structure consisting of GaN and AlGaN stacked one on another.Since the p-type cladding layer 111 formed in the super latticestructure increases the proportion of Al in the entire cladding layer,refractive index of the cladding layer can be decreased. Also becausethe band gap energy can be increased, it is very effective in reducingthe threshold. Moreover, since pits generated in the cladding layer canbe reduced by the super lattice structure compared to a case withoutsuper lattice structure, occurrence of short-circuiting is also reduced.

(p-Type Contact Layer 112)

Last, at a temperature of 1050° C., a p-type contact layer 112 made ofp-type GaN doped with Mg in a concentration of 1×10²⁰/cm³ is formed to athickness of 150 Å on the p-type cladding layer 111. The p-type contactlayer may be formed from p-type In_(x)Al_(y)Ga_(1-x-y)N (0≦X, 0≦Y,X+Y≦1) and preferably from Mg-doped GaN which achieves the best ohmiccontact with the p-type electrode 20. Since the contact layer 112 is thelayer where the electrode is to be formed, it is desirable to have ahigh carrier concentration of 1×10¹⁷/cm³ or higher. When theconcentration is lower than 1×10¹⁷/cm³, it becomes difficult to achievesatisfactory ohmic contact with the electrode. Forming the contact layerin a composition of GaN makes it easier to achieve satisfactory ohmiccontact with the electrode. After the reaction has finished, the waferis annealed in nitrogen atmosphere at 700° C. in the reaction vesselthereby to further decrease the electrical resistance of the p-typelayer.

After forming the nitride semiconductor layers one on another asdescribed above, the wafer is taken out of the reaction vessel. Then aprotective film of SiO₂ is formed on the surface of the top-most p-typecontact layer, and the surface of the n-type contact layer 104 whereonthe n-type electrode is to be formed is exposed as shown in FIG. 8 byetching with SiCl₄ gas in the RIE (reactive ion etching) process. Forthe purpose of deep etching of the nitride semiconductor, SiO₂ is bestsuited as the protective film. At the same time the n-type contact layer104 is exposed, end faces of the active layer which would become theresonance end face may also be exposed thereby making the etched endface serve as the resonance end face.

Now a method for forming the first waveguide region C₁ and the secondwaveguide region C₂ as the stripe waveguide region will be described indetail below. First, a first protective film having thickness of 0.5 μmis formed from Si oxide (mainly SiO₂) over substantially the entiresurface of the top-most p-type contact layer (upper contact layer) 8 bymeans of a PDP apparatus. Then the first protective film 161 is formedby patterning (refer to FIG. 5A used in the description of theembodiment). Patterning of the first protective film 161 is carried outby means of photolithography process and the RIE (reactive ion etching)apparatus which employs SiF₄ gas. Then by using the first protectivefilm 161 as the mask, a part of the p-type contact layer 112 and thep-type cladding layer 111 is removed so that the p-type cladding layer111 remains with a small thickness on both sides of the mask, therebyforming striped ridges over the active layer 3 (refer to FIG. 5B used inthe description of the embodiment). This results in the second ridge 202which constitutes the second waveguide region C₂ being formed. At thistime, the second ridge is formed by etching a part of the p-type contactlayer 112 and the p-type cladding layer 111, so that the p-type claddinglayer 111 is etched to a depth of 0.01 μm.

After forming the striped second ridge, a photoresist film is formed asthe third protective film 163 except for a part of the second ridge (theportion which constitutes the first waveguide region) (refer to FIG. 5Cused in the description of the embodiment). The first protective film161 remains on the top surface of the ridge in the portion where thesecond waveguide region is to be formed and on the top surface of theridge in the portion where the first waveguide region is to be formed.

Then after transferring to the RIE (reactive ion etching) apparatus, thethird protective film 163 and the first protective film 161 are used asthe masks to etch on both sides of the first protective film 161 in theportion where the first waveguide region is to be formed to such a depthas the n-type cladding layer 106 is exposed by using SiF₄ gas, therebyto form the first ridge of stripe configuration which constitutes thefirst waveguide region C₁. At this time, the first ridge formed in thestripe configuration is formed by etching the n-type cladding layer 106on both sides of the first ridge to such a depth as the thicknessbecomes 0.2 μm.

The wafer having the first waveguide region C₁ and the second waveguideregion C₂ formed thereon is then transferred to the PVD apparatus, wherethe second protective film 162 made of Zr oxide (mainly ZrO₂) with athickness of 0.5 μm continuously on the surface of the first protectivefilm 161, on the side faces of the first and second ridges, on thep-type cladding layer 111 which is exposed by etching and on the n-typecladding layer 106 (refer to FIG. 7A used in the description of theembodiment).

After forming the second protective film 162, the wafer is subjected toheat treatment at 600° C. When the second protective film is formed froma material other than SiO₂, it is preferable to apply heat treatment ata temperature not lower than 300° C., preferably 400° C. or higher butbelow the decomposition temperature of the nitride semiconductor (1200°C.) after forming the second protective film, which makes the secondprotective film less soluble to the material (hydrofluoric acid) whichdissolves the first protective film.

Then the wafer is dipped in hydrofluoric acid to remove the firstprotective film 161 (lift off process). Thus the first protective film161 provided on the p-type contact layer 112 is removed thereby exposingthe p-type contact layer 112. The second protective film 162 is formedon the side faces of the first ridge 201 and the second ridge 202 whichare formed in stripes on the first waveguide region C₁ and the secondwaveguide region C₂, and on the surface located on both sides of theridge continuing thereto (the surface of the p-type cladding layer 111located on both side of the second ridge and the surface of the n-typecladding layer located on both side of the first ridge) by the processdescribed above (refer to FIG. 7C used in the description of theembodiment).

After the first protective film 161 provided on the p-type contact layer112 is removed as described above, a p-type electrode 120 made of Ni/Auis formed on the surface of the exposed p-type contact layer makingohmic contact therewith. The p-type electrode 120 is formed with stripewidth of 100 μm over the second protective film 162 as shown in FIG. 8.At this time, the p-type electrode 120 is formed only in the firstwaveguide region C₁ and the second waveguide region C₂ in the directionof stripe in the first embodiment. In the first embodiment, the p-typeelectrode 120 is formed to such a length that does not reach both endsof the second waveguide region C₂. After forming the second protectivefilm 162, an n-type electrode 21 made of Ti/Al is formed in a directionparallel to the stripe on the n-type contact layer 104 which has beenalready exposed.

Then the region where lead out electrodes for the p-type and n-typeelectrodes are to be formed is masked, and a multi-layered dielectricfilm 164 made of SiO₂ and TiO₂ are formed. With the mask being removed,apertures for exposing the p-type and n-type electrodes are formed inthe multi-layered dielectric film 164. Through the apertures, thelead-out electrodes 122, 123 made of Ni—Ti—Au (1000 Å-1000 Å-8000 Å) areformed on the p-type and n-type electrodes. In the first embodiment, theactive layer 108 in the second waveguide region C₂ is formed with awidth of 200 μm (width in the direction perpendicular to the resonatordirection). The guide layer is also formed with a similar width.

After forming the p-type and n-type electrodes, the resonance end facesare formed on the ends of the first waveguide region C₁ and the secondwaveguide region C₂ by etching further till the substrate is exposed.

In the laser device of the first embodiment, the resonator was formedwith total length of 650 μm and the first waveguide region C₁ was formedwith total length of 5 μm including one of the end faces of theresonator. Thus the second waveguide region C₂ has total length of 645μm including the other end face. On the end faces of the resonator whichare formed by etching, the multi-layered dielectric film made of SiO₂and TiO₂ was formed. Then sapphire substrate of the wafer is polished toa thickness of 70 μm and is divided from the substrate side into barshape with the wafer of bar shape further divided into individualdevices, thereby to obtain the laser devices.

While the resonance end face is formed by forming the multi-layereddielectric film on the etched surface in the first embodiment, the wafermay be divided into bar shape along (11-00) M surface which is cleavedsurface of GaN to use the surface as the resonance end face.

With the laser device of the first embodiment fabricated as describedabove, continuous oscillation at wavelength 405 nm with an output powerof 30 mW was confirmed with threshold of 2.0 kA/cm² at room temperature.Also good beam of F.F.P. was obtained with aspect ratio of 1.5,indicating satisfactory beam characteristics for the light source of anoptical disk system. The excellent characteristics are achieved throughsuch features of the present invention that laser beam of desiredoptical characteristics can be emitted by adjusting the width of theridge of the first waveguide region C₁ on the light emitting sideregardless of the stripe width of the second waveguide region C₂ whichfunctions mainly as a gain region. Also the laser device of the firstembodiment does not experience a shift in the transverse mode, in theoptical output range from 5 to 30 mW, and therefore has favorablecharacteristic suitable for reading and writing, light source of anoptical disk system. In addition, the laser device having goodperformance when driven with 30 mW comparable to the conventionalrefractive index guided laser device.

Also in the first embodiment, the p-type electrode may be provided overa length that covers the first waveguide region C₁ as shown in FIG. 7C.With this constitution, too, the laser device having excellent beamcharacteristics and long service life can be made.

Embodiment 2

The laser device is fabricated similarly to the first embodiment exceptfor the length of the first waveguide region C₁ which is set to 1 μm. Inorder to form the first waveguide region C₁ with such a small length,the first ridge of stripe shape is formed longer than the final lengthof the resonator (for example, several tens to about 100 μm) and thenthe resonance end face is formed by etching or dividing, the substrateat such a position as the desired length of the first waveguide regionC₁ is obtained. As a result, it becomes more difficult to form thesecond ridge 201 with stable shape than in the case of the firstembodiment, although the transverse oscillation mode can be wellcontrolled even with this length. Also shorter length of the firstwaveguide region improves the device life slightly over the firstembodiment.

Embodiment 3

The laser device of the third embodiment is constituted similarly to thefirst embodiment except for forming the first waveguide regions C₁having length of 5 μm at both ends thereof (refer to FIG. 4B). That is,the laser device of the third embodiment has the second waveguide regionC₂ located at the center and the first waveguide regions C₁ located onboth sides of the former, while the first waveguide region C₁ includesthe resonator end face. The laser device of the third embodiment havingsuch a constitution has both F.F.P. and aspect ration of the beamsimilar to those of the first embodiment.

Embodiment 4

The laser device is constituted similarly to the first embodiment exceptthat the second ridge 202 provided to constitute the second waveguideregion C₂ is formed by etching to leave the p-type guide layer having athickness of 500 Å on both sides of the second ridge. While laser devicethus obtained has lower threshold than that of the first embodiment,beam characteristics similar to those of the first embodiment areobtained.

Embodiment 5

The laser device of the fifth embodiment is constituted similarly to thefirst embodiment except for providing a slant surface between the firstwaveguide region C₁ and the second waveguide region C₂ (refer to FIG.4A).

Specifically, in the fifth embodiment, in the boundary between the firstwaveguide region C₁ and the second waveguide region C₂, the sectionalsurfaces formed by etching between the surface of the n-type claddinglayer 106 located on both sides of the first ridge and the surface ofthe p-type cladding layer 111 located on both sides of the second ridgeis inclined to 90° with respect to the surface of the n-type claddinglayer 106.

Though the laser device manufactured as described above may be subjectedto variations in the device characteristics compared to the firstembodiment, the effect of the present invention that goof F.F.P. isobtained and the reliability is improved can be achieved.

Embodiment 6

The laser device of the sixth embodiment is constituted similarly to thefirst embodiment except for providing the third waveguide region C₃between the first waveguide region. C₁ and the second waveguide region Cas shown in FIG. 13. Specifically, in the laser device of the sixthembodiment, after the second ridge 202 to a depth reaching the layer ofsecond conductivity type (p-type cladding layer 111), the thirdwaveguide region C₃ having a side face 204 formed to have an angle α of20° from the resonator direction AA is formed at the same time when thefirst ridge is formed by etching down to the layer of first conductivitytype (n-type cladding layer 106). Thus the laser device of the sixthembodiment which has the third waveguide region C₃ in addition to thefirst waveguide region C₁ and the second waveguide region C₂ is made. Inthe laser device of the sixth embodiment constituted as described above,light which has been guided while spreading in the active layer plane inthe second waveguide region C₂ is reflected on the side face 204 of thethird waveguide region C₃ and is directed toward the first waveguideregion C₁, and therefore the light can be guided smoothly. That is, asthe light guided in the direction of the resonator falls on the sideface 204 with an incident angle of (90°−α), the light undergoes totalreflection on the side face 204 and can be guided into the stripewaveguide region without loss. In the second waveguide region, C₂ andthe third waveguide region C₃, effective difference in the refractiveindex is provided in the active layer plane by means of the second ridge202 which is provided on the layer of second conductivity type (p-typecladding layer 111), and the stripe waveguide region is formed. In thethird waveguide region C₃, light guided while coming out of the regionright below the second ridge can be guided satisfactorily into the firstwaveguide region C₁.

In the sixth embodiment, as described above, since the side face 204 isinclined against the side face of the first ridge 201 in the firstwaveguide region C₁, light can be smoothly guided. The boundary betweenthe side face 204 and the second waveguide region C₂ may also beconnected directly to the second waveguide region C₂ without bending asshown in FIG. 13.

In the laser device of the sixth embodiment, as described above, sincelight guided in the stripe waveguide region in the active layer plane orcoming out thereof in the second waveguide region C₂ can be efficientlyguided into the first waveguide region C₁, the device characteristicscan be improved. In the laser device of the sixth embodiment, inparticular, threshold of current density can be decreased and slopeefficiency can be improved.

Embodiment 7

The laser device of the seventh embodiment is constituted similarly tothe first embodiment except for constituting the first waveguide regionC₁ in 2-step configuration where the side face is formed in two steps.

Specifically, in the seventh embodiment, after forming the striped ridgeby etching to such a depth that does not reach the active layer, a ridgewider than the stripe width of the ridge is etched down to the n-typecladding layer 106 in a portion where the first waveguide region is tobe formed, thereby to form the 2-step ridge.

FIG. 14A is a perspective view showing the laser device structure of theseventh embodiment, FIG. 14C is a sectional view of the first waveguideregion C₁ and FIG. 14B is a sectional view of the second waveguideregion C₂. In the laser device of the seventh embodiment, as shown inFIG. 14A, the first waveguide region C₁ is formed in the form of 2-stepridge comprising an upper ridge of width S_(w1) and a lower ridge ofwidth S_(w2). In the first waveguide region C₁, since the active layeris located in the lower ridge, and width of the active layer 3 isdetermined by the width S_(w2) of the lower ridge, the waveguide can beconsidered to be formed substantially by the lower ridge. The structureof the seventh embodiment makes it easier to control the width S_(w2) ofthe lower ridge compared to a case where the first ridge is formed as inthe first embodiment or the like and, as a result, width of the activelayer of the first waveguide region can be formed accurately. This isbecause, while etching is carried out in two steps with a single maskwhen the first ridge 201 for constituting the first waveguide region C₁is formed by the method shown in FIG. 5, a step is formed in theboundary between the portion shared by the second ridge which has beenformed first and the portion below thereof during the second etching tosuch a depth that reaches the layer of first conductivity type, thusmaking it unreliable to accurately control the width of the lowerportion.

According to the seventh embodiment, in contrast, after etching theupper ridge in the etching process common to the second ridge, the lowerridge is formed through etching by making and using a mask differentfrom the mask used when forming the upper, ridge. Consequently, thelower ridge can be formed with accurate width while the active layer 3located in the lower ridge can also be formed with accurate width.

Thus according to this embodiment, laser device having characteristicsequivalent to the first embodiment can be manufactured with lessvariations due to manufacturing. In other words, the laser device of theseventh embodiment is advantageous with respect to manufacturing.

Embodiment 8

The laser device structure of the eighth embodiment has the thirdwaveguide region formed between the first waveguide region and thesecond waveguide region, with the third waveguide region beingconstituted differently from the sixth embodiment.

Specifically, in the laser device structure of the eighth embodiment,the third waveguide region C₃ is constituted from a third ridge providedon the p-type cladding layer 111 and the p-type contact layer 112 asshown in FIG. 15A, with the third ridge decreasing in width toward thefirst waveguide region.

Thus according to the eighth embodiment, forming the third waveguideregion makes it possible to connect the first waveguide region and thesecond waveguide region, which have different widths, without changingthe width of the waveguide discretely.

FIG. 15A is a perspective view showing the laser device structure of theeighth embodiment, and FIG. 15B is a cross sectional view of the activelayer. In FIG. 15B, width S_(w1) is the width of the second ridge at thebase thereof, and width S_(w2) is the width of the active layer portionof the first ridge.

The imaginary line (a dash and two dots line) in FIG. 15B is theprojection of the second ridge and the third ridge onto the crosssectional plane of the active layer. Since the waveguides of the secondwaveguide region and the third waveguide region are constituted byproviding the effective difference in the refractive index in the activelayer corresponding to the second ridge and the third ridge, theimaginary line (a dash and two dots line) can be considered tosubstantially represent the waveguides of the second waveguide regionand the third waveguide region.

The laser device structure of the eighth embodiment manufactured asdescribed above shows excellent characteristics similarly to that of thefirst embodiment.

Embodiment 9

The ninth embodiment is an example of manufacturing the laser devicewhich is constituted similarly to the first embodiment by a methoddifferent from the first embodiment.

In the ninth embodiment, the second ridge is formed after the firstridge has been formed.

Specifically, after forming the layers one on another similarly to thefirst embodiment, the first protective film 161 having stripe shape isformed on the surface of the laminate as shown in FIG. 5A. Then as shownin FIG. 6A, the third protective film 163 is formed except for a part ofthe first protective film 161 (where the first waveguide region is to beformed), and both sides of the first protective film 161 are etched tosuch a depth as the lower cladding layer 5 (n-type cladding layer 106)is exposed, thereby to form the first ridge 201 as shown in FIG. 6B.Then after temporarily removing the third protective film 163, the thirdprotective film 163 is formed to cover the first ridge 201 as shown inFIG. 6C. Under this condition, portions where the second waveguideregion is to be formed except for those on both sides of the firstprotective film 161 are covered by at least one of the first protectivefilm and the third protective film 163. After creating this state, thesecond ridge is formed by etching the regions which are not covered bythe first protective film 161 and the third protective film 163 to sucha depth that does not reach the active layer.

At this time, width and height of ridges that constitute the firstwaveguide region C₁ and the second waveguide region C₂ are set tosimilar values as the first embodiment. Then the third protective film163 provided on the first waveguide region C₁ is removed to leave onlythe first protective film 161 which is the striped mask, followed by asubsequent process similar to the first embodiment wherein the second,protective film (buried layer) is formed on the side face of the stripeand on the surface of the nitride semiconductor layer which continuestherefrom. Then the laser device is obtained similarly to the firstembodiment. According to the method of the ninth embodiment describedabove, although the number of processes increases compared to the methodof the first embodiment, laser device similar to that of the firstembodiment can be manufactured.

Embodiment 10

The tenth embodiment is an example of manufacturing the laser device byusing a nitride semiconductor substrate, with the basic deviceconstitution having the second waveguide region C₂ of the structureshown in FIG. 8 and the first waveguide region C₁ of the structure shownin FIG. 9.

(Substrate 101)

In the tenth embodiment, the nitride semiconductor substrate made of GaN80 μl m thick which is fabricated as follows is used.

As the substrate of different material where on the nitridesemiconductor is to be grown, a sapphire substrate measuring 425 μm inthickness and 2 inches in diameter with the principal plane lying on theC plane and orientation flat surface on the A plane is prepared. Thewafer is set in a MOCVD reaction vessel. Then with the temperature setto 510° C. and using hydrogen as the carrier gas and ammonia and TMG(trimethyl gallium) as the stock material gas, a low-temperature growthbuffer layer made of GaN is formed to a thickness of 200 Å on thesapphire substrate, followed by the growth of a base layer made ofundoped GaN is grown to a thickness of 2.5 μm by using TMG and ammoniaas the stock material gas, with the temperature set to 1050° C. Aplurality of masks made of SiO₂ and in the shape of stripe each 6 μmwide are formed in parallel to each other, in a direction of θ=0.3° fromthe direction perpendicular to the orientation flat surface (A plane) ofthe sapphire substrate, so that the interval between masks (aperture ofthe mask) is 14 μm. Then the substrate is returned to the MOCVDapparatus where the undoped GaN is grown to a thickness of 15 μm. Inthis process, GaN which is grown selectively through the mask aperturegrows mainly in the longitudinal direction (thickness direction) in themask aperture, and grows in the lateral direction over the mask, so thatthe base layer covering the mask and the mask aperture is formed. In thebase layer which has grown as described above, occurrence of throughdislocation in the nitride semiconductor layer that has grown laterallycan be decreased. Specifically, through dislocation occurs in such a wayas the dislocation density increases to about 10¹⁰/cm² over the maskaperture and around the center of the mask where fronts of growingnitride semiconductor bodies approaching laterally from both sides ofthe mask join, and the dislocation density decreases to about 10⁸/cm²over the mask except for the central portion thereof.

Then the wafer is placed in the HVPE apparatus where undoped GaN isgrown to a thickness of about 100 μm on the base layer (the layer grownto about 100 μm thick will be referred to as thick film layer). Then thesubstrate of different material, the low-temperature growth bufferlayer, the base layer and a part of the thick film layer are removedthereby to leave only the thick film layer (singularization) and obtaina GaN substrate 80 μm thick. Although the thick film layer formed by theHVPE may be made of a nitride semiconductor other than GaN, it ispreferable to use GaN or AlN which makes it possible to easily growthick nitride semiconductor layer with good crystallinity, according tothe present invention. The substrate of different material may beremoved either after forming the device structure which will bedescribed later, or after forming the waveguide, or after forming theelectrode. When the substrate of different material is removed beforecutting the wafer into bars or chips, cleavage planes of the nitridesemiconductor ({11-00} M plane, {1010} A plane, {0001} C planeapproximated by hexagonal system) can be used when cutting or cleavinginto chips.

(Base Layer 102)

A base layer 102 is formed to a thickness of about 15 μm on the nitridesemiconductor substrate so as to grow in the lateral direction as well.By using a striped SiO₂ mask similarly to the base layer used whenfabricating the nitride semiconductor substrate.

[Buffer Layer 103]

A buffer layer 103 made of undoped AlGaN with Al proportion of 0.01 isformed on the base layer 102. Although the buffer layer 103 may beomitted, in case the substrate which uses lateral growth is made of GaN,or in case the base layer formed by using lateral growth is made of GaN,it is preferable to form the buffer layer 103 since the occurrence ofpits can be decreased by using the buffer layer 103 made of a nitridesemiconductor which has lower thermal expansion coefficient than tat ofGaN, namely Al_(a)Ga_(1-a)N (0<a≦1) or such material. That is, pits arelikely to occur when a nitride semiconductor is grown on other type ofnitride semiconductor which has been grown in a process accompanied bylateral growth as in the case of the base layer 102, while the bufferlayer 103 has an effect of preventing the occurrence of pits.

It is also preferable that the proportion a of Al contained in thebuffer layer 103 is 0<a<0.3, which makes it possible to form a bufferlayer of good crystallinity. After forming the buffer layer 103, ann-type contact layer of composition similar to that of the buffer layermay be formed, thereby giving the buffer effect also to the n-typecontact layer 104. That is, the buffer layer 103 decreases the spits andimproves the device characteristics when at least one layer thereof isprovided between the laterally grown layer (GaN substrate) and thenitride semiconductor layer which constitutes the device structure, orbetween the active layer within the device structure and the laterallygrown layer (GaN substrate), and more preferably on the substrate sidein the device structure, between the lower cladding layer and thelaterally grown layer (GaN substrate). When a buffer layer which alsoperforms the function of the n-type contact layer, proportion a of Alcontained therein is preferably within 0.1 so as to obtain good ohmiccontact with the electrode. The buffer layer formed on the base layer102 may be grown at a low temperature in a range from 300 to 900° C.similarly to the buffer layer which is provided on the substrate ofdifferent material described above, the effect of reducing the pits canbe improved by single crystal growth at a temperature in a range from800 to 1200° C. Moreover, the buffer layer 103 may be either doped withn-type or p-type impurity or undoped, although it is preferable to growwithout doping in order to obtain good crystallinity. In case two ormore layers of buffer layer are provided, the layer can be formed whilechanging the concentration of n-type or p-type impurity and/or theproportion of Al.

(n-Type Contact Layer 104)

The n-type contact layer 104 made of Al_(0.01)Ga_(0.99)N doped with Siin a concentration of 3×10¹⁸/cm³ is formed to a thickness of 4 μm on thebuffer layer 103.

(Crack Preventing Layer 105)

A crack preventing layer 105 made of In_(0.06)Ga_(0.94)N is formed to athickness of 0.15 μm on the n-type contact layer 104.

(n-Type Cladding Layer 106)

An n-type cladding layer 106 of super lattice structure to totalthickness of 1.2 μm on the crack preventing layer 105.

Specifically, the n-type cladding layer 106 is formed by forming undopedIn_(0.05)Ga_(0.95)N to a thickness of 25 μm and GaN layer doped with Siin a concentration of 1×10¹⁹ cm³ alternately one on another.

(n-Type Optical Guide Layer 107)

An n-type optical guide layer 107 made of undoped GaN of a thickness of0.15 μm is formed on the n-type cladding layer 106.

(Active Layer 108)

The active layer 108 of multiple quantum well structure with totalthickness of 550 Å on the n-type optical guide layer 107.

Specifically, the active layer 108 is formed by forming the barrierlayer (B) made of In_(0.05)Ga_(0.95) doped with Si in a concentration of5×10¹⁸/cm³ with a thickness of 140 Å and a well layer (W) made ofundoped In_(0.13)Ga_(0.87)N with a thickness of 50 Å alternately in theorder of (B)-(W)-(B)-(W)-(B).

(p-Type Electron Confinement Layer 109)

The p-type electron confinement layer 109 made of p-typeAl_(0.3)Ga_(0.7)N doped with Mg in a concentration of 1×10²⁰/cm³ isformed to a thickness of 100 Å on the active layer 108.

(p-Type Optical Guide Layer 110)

The p-type optical guide layer 110, made of p-type GaN doped with Mg ina concentration of 1×10¹⁸/cm³ is formed to a thickness of 0.15 μm on thep-type electron confinement layer 109.

(p-Type Cladding Layer 111)

A p-type cladding layer 111 of super lattice structure with totalthickness of 0.45 μm is formed on the optical guide layer 110.

Specifically, the p-type cladding layer 111 is formed by forming undopedAl_(0.05)Ga_(0.95)N of thickness 25 Å and p-type GaN layer doped with Mgin a concentration of 1×10²⁰/cm³ of thickness 25 Å alternately one onanother.

(p-Type Contact Layer 112)

The p-type contact layer 112 made of p-type GaN doped with Mg in aconcentration of 2×10²⁰/cm³ is formed to a thickness of 150 Å on thep-type cladding layer 111.

After forming the device structure from the n-type contact layer 104 tothe p-type contact layer 112 as described above, the n-type contactlayer 104 is exposed, the 31 and the second waveguide region C₂ areformed by etching, and the second protective film 162 (buried layer) isformed on the side faces of the first ridge and the second ridge and onthe nitride semiconductor layer surface which continues thereto,similarly to the first embodiment. At this time, the second ridgeprovided for constituting the second waveguide region C₂ is formed byetching the p-type optical guide layer 110 on both sides of the secondridge to such a depth as the film thickness becomes 0.1 μm.

Now the method for forming the resonating end face of the laser deviceaccording to the tenth embodiment will be described below.

In the tenth embodiment, the resonating end faces are formed efficientlyby disposing a pair of laser devices so that the two devices oppose eachother in a symmetrical arrangement with respect to a plane of symmetry.

Specifically, the second waveguide regions C₂ each 645 μm in length areformed on both sides of the first waveguide region C₁ which is 10 μmlong (the first waveguide regions of a pair of laser devices coupled)(refer to FIG. 17B at portions IIIb and IVb).

The outer end faces of the second waveguide regions C₂ on both sidesthereof are formed at the same time as the etching for exposing then-type contact layer.

Then similarly to the first embodiment, the n-type electrode 121 and thep-type electrode 120 are formed on the surfaces of the n-type contactlayer 104 and the p-type contact layer 112.

Then an insulation film (reflector film) 164 made of a dielectricmulti-layered film is formed over the entire surfaces which are exposedincluding the end faces of the second waveguide region and the sidefaces of each ridge provided for constituting the waveguide regions.

This process forms the insulation film 164 which functions as areflector film at the end face of the second waveguide region C₂ andfunctions as an insulation film in other parts (particularly functionsto prevent short-circuiting between p-n electrodes). In the tenthembodiment, the p-type electrode 120 is formed on a part of the p-typecontact layer 112 with a width smaller than the stripe width of thep-type contact layer 112, unlike those shown in FIGS. 8 and 9. Thep-type electrode 120 is formed only on the top of the second waveguideregion C₂ in the direction of stripe. The p-type electrode 120 is formedat a small distance from the end of the second waveguide region C₂.

Then a part of the insulation film 164 provided on the n-type and p-typeelectrodes is removed to expose the electrodes, thereby to form padelectrodes 122, 123 which make electrical connection on the surfaces ofthe electrodes.

Then at around the center of the first waveguide region C₁ which is 10μm (refer to line E-E in FIG. 17B), the nitride semiconductor is cleavedalong M surface into bar shape, and the bars are cleaved in parallel tothe resonator direction along A plane perpendicular to the M plane ofcleaving between the devices, thereby to obtain chips.

The laser chip obtained as described above has the first waveguideregion C₁ having length of about 5 μm and the second waveguide region C₂having length of 645 μm, with the end face of the first waveguide regionC₁ being used as the light emitting side, similarly to the firstembodiment.

The laser device obtained as described above has threshold currentdensity of 2.5 kA/cm² and threshold voltage of 4.5V at room temperature,with oscillation wavelength of 405 nm and aspect ratio of 1.5 for thelaser beam emitted. With continuous oscillation at 30 mW, the laserdevice can operate with a high output power for 1000 hours or longer.The laser device is capable of continuous oscillation in an output rangefrom 5 mW to 80 mW, and has beam characteristics suited as the lightsource for optical disk systems in this output range.

Embodiment 11

The laser device of the eleventh embodiment is constituted by usingSi-doped n-type GaN which is 80 μm thick as the substrate 101 instead ofthe undoped GaN which is 80 μm thick of the tenth embodiment, Thesubstrate 101 made of Si-doped n-type GaN is made by forming a lowtemperature growth buffer layer on a substrate of different material,forming a base layer in a growing process which is accompanied bylateral growth, forming a thick film of Si-doped n-type GaN to athickness of 100 μm by HVPE, and then removing the substrate ofdifferent material.

In the eleventh embodiment, the buffer layer 103 made of Si-dopedAl_(0.01)Ga_(0.99)N is formed on the n-type GaN substrate 101, andthereon the layers are formed one on another from the n-type contactlayer 104 to the p-type contact layer 112 similarly to the firstembodiment.

Then a separation groove is formed by etching so as to expose thesurface of the p-type contact layer 112 in order to define the regionwhere the waveguide regions of the deices are to be formed. In theeleventh embodiment, unlike the first embodiment, it is not necessary toprovide a space for forming the n-type electrode on the exposed surfaceof the n-type contact layer in order to make a structure of opposingstructure of electrodes on both sides of the substrate without forming apair of positive and negative electrodes on the same side. Therefore,adjacent devices can be disposed nearer to each other than in the caseof the tenth embodiment.

In the eleventh embodiment, different regions are defined by exposingthe n-type contact layer by etching, but the following process may alsobe carried out without etching for achieving opposing arrangement inthis constitution. When forming the separation groove, the layer betweenthe n-type contact layer and the substrate may be exposed, or theseparation groove may be formed so as to expose the substrate. Moreover,in case the separation groove is formed by exposing the substrate, thesubstrate may be etched midway thereby to expose the substrate.

The regions for defining the devices may not be necessarily formed foreach device, and a region to constitute two devices collectively may beformed, as described in the tenth embodiment, or a region to constitutethree devices collectively may be formed (for example, portions III andIV shown in FIGS. 17A, 17B are formed collectively).

Similarly in the direction perpendicular to the light guiding direction,a plurality of regions may be formed continuously without formingseparation grooves between the devices.

Cracking and chipping in the active layer due to the impact of divisioncan be avoided by forming the groove by etching deeper than the activelayer and dividing along the groove (for example, portion A-A shown inFIG. 17A, FIG. 17B).

In the eleventh embodiment, the region for each device is separated tomake the individual devices. Then similarly to the tenth embodiment, thestripe ridges for constituting the waveguide regions are formed and thenthe first waveguide region C₁ and the second waveguide region C₂ areformed in each region, corresponding to each device. The first waveguideregion C₁ is formed with stripe length of 10 μm.

Then similarly to the tenth embodiment, a p-type electrode of stripeshape having a width smaller than the width of the p-type contact layeris formed on the surface of the p-type contact layer only in the secondwaveguide region C₂. At this time the p-type electrode of stripe shapeis formed in such a length that does not reach the end face of thesecond ridge which constitutes the second waveguide region C₂ so as tokeep away a little therefrom.

Then an n-type electrode is formed on the back side of the substrate(the surface which opposes the substrate surface whereon devicestructure is formed), Then similarly to the tenth embodiment, theinsulation film (reflector film) 164 made of dielectric multi-layeredfilm is formed over substantially the entire surface on the side of thesubstrate where the device structure is formed and, with a part of thep-type electrode being exposed, a pad electrode is formed so as toelectrically connect to the exposed p-type electrode.

Last laser devices in the form of chips are obtained by cleaving at theD-D cutting position located substantially at the center of the firstwaveguide region C₁ as the cutting direction perpendicular to theresonator and along the M plane of the substrate at the A-A cuttingposition between the devices thereby to separate into bars and thencleaving between the devices along A plane perpendicular to the cleavageplane.

The laser device obtained as described above has the cleavage surface atthe end of the first waveguide region C₁ and the etched end face whereonthe reflector film is provided at the end of the second waveguide regionC₂ as the resonance end faces, and is capable of laser oscillation. Thelaser device obtained as described above has excellent lasercharacteristics similar to those of the tenth embodiment.

Embodiment 12

The laser device of the twelfth embodiment is made by forming theresonator end faces simultaneously as etching down to the n-type contactlayer, and dividing the substrate between the resonator end faces alongthe AA cut surface in I and II of FIG. 17A after etching down to thesubstrate, in the eleventh embodiment. At this time, the dimension ofthe portion protruding from the resonator end face is set to 3 μm. Thelaser device obtained as described above has excellent lasercharacteristics similar to the device characteristics and opticalcharacteristics of the eleventh embodiment.

Comparative Embodiment 1

As a first comparative embodiment, a laser device having the secondwaveguide region C₂ formed over the entire length thereof withoutforming the first waveguide region C₁ in the first embodiment isfabricated.

In the first comparative embodiment, different layers which constitutethe device structure are stacked one on another similarly to the firstembodiment. Then as shown in FIG. 5B, the second stripe ridge is formedto extend from one end face of the device to the other end face, byusing the first protective film 161 as the mask.

Then a protective film made of ZrO₂ is formed on the side face of thefirst ridge formed over the entire length thereof and on the surfaces onboth sides thereof which are exposed by etching. The wafer is thendipped in hydrofluoric acid thereby to remove the first protective film161 by lift off. Then similarly to the first embodiment, the resonanceend face and the electrodes are formed thereby to obtain the laserdevice of the first comparative embodiment which has only the secondridge for constituting the second waveguide region C₂.

In the laser device of the first comparative embodiment fabricated asdescribed above, it is difficult to effectively suppress the unnecessarytransverse mode, thus resulting in lower stability of the transversemode and frequent occurrence of kink in the current optical outputcharacteristic.

Particularly in a high output range of large optical output power, forexample, output power of 30 mW which is required to write data in anoptical disk system, shift of the transverse mode is likely to occur.Also because the device characteristics are sensitive to the dimensionalaccuracy of the second ridge of stripe shape, significant variationsoccur among the devices thus making it difficult to improve the yield ofproduction as shown in FIG. 10. The aspect ratio of the laser beam spotmostly fall within a range from 2.5 to 3.0, which means significantlylow yield of production provided that the criteria of acceptance foraspect ratio is 2.0 or lower.

Now the result of investigation conducted to verify the effects of theconstitution of the laser device according to the present invention(service life of laser device, drive current and controllability, oftransverse mode) will be described below.

In the investigation, device constitution (laminated structure ofsemiconductor) similar to the first embodiment was used to fabricate thelaser devices of different ridge height while changing the depth ofetching, and the service life of laser device, drive current andcontrollability of transverse mode were evaluated on the laser devices.

FIG. 12 shows the service life of the laser device (tested with opticaloutput power of 30 mW) for different depths of etching.

As shown in FIG. 12, when etching is carried out to a depth near theboundary of the p-type cladding layer and the p-type optical guidelayer, device life becomes longest but the life becomes shorter when theetching depth is smaller. Also when etching near to the boundary of thep-type cladding layer and the p-type optical guide layer, the laserdevice decreases abruptly, indicating that there occurs an significantlyadverse influence on the device life when the stripe waveguide region isformed by etching to a depth that reaches the active layer. When thedevice life is taken into consideration, therefore, it is better to etchto a depth which does not reach the p-type electron confinement layer.Also it can be understood that, when the ridge is formed by etching to adepths in a range of 0.1 μm above and below the boundary between thep-type cladding layer and the p-type optical guide layer, very longservice life is obtained. When the confinement of light in the directionof thickness is taken into consideration, it is preferable to etch tosuch a depth which does not reach the p-type guide layer. With thisrespect, it is more preferable to carry out etching to a depth of 0.1 μmabove the interface of the p-type cladding layer and the p-type opticalguide layer.

FIG. 10 is a graph showing the acceptance ratio for different depths ofetching. From FIG. 10, it can be seen that a high acceptance ratio canbe achieved by etching to a depth deeper than a point 0.1 μm above theinterface of the p-type cladding layer and the p-type optical guidelayer. The acceptance ratio shown in FIG. 10 indicates what proportionof devices which have proved capability to oscillate can oscillate inthe fundamental single transverse mode at 5 mW, while the stripe widthof the waveguide region at this time was 1.8 μm.

When etched to such a depth as 0.1 μm or more of the p-type claddinglayer remains on both sides of the ridge, kinks occur abruptly thusleading to a significant decrease in the acceptance ratio.

FIG. 11 shows the drive voltage (with optical output of 30 mW) as afunction of the depth of etching, with the width of the waveguide regionbeing set to 1.8 μm for the investigation. As will be clear from FIG.11, the drive current remains constant at. 50 mA regardless of the depthof etching, when etching is carried out deeper than the mid point of thep-type optical guide layer (mid point in the direction of thickness) onthe active layer side. When the depth of etching is decreased from themid point of the p-type optical guide layer, the current graduallyincreases up to 0.1 μm above the boundary of the p-type cladding layerand the p-type optical guide layer, while the current sharply increaseswhen the depth of etching is shallower than 0.1 μm above the boundary ofthe p-type cladding layer and the ft type optical guide layer (such adepth of etching that a thickness of 0.1 μm or more of the p-typecladding layer remains on both sides of the ridge). When etched to sucha depth as thickness of 0.25 μm or more of the p-type cladding layerremains, it becomes impossible to achieve an optical output of 30 mW.

Comparative Embodiment 2

As a second comparative embodiment, a laser device having the firstwaveguide region formed over the entire length thereof without formingthe second waveguide region in the first embodiment is fabricated.

In the second comparative embodiment, different layers which constitutethe device structure are stacked one on another similarly to the firstembodiment. Then as shown in FIG. 5A, the ridge of stripe shape whichconstitutes the first waveguide region C₁ is formed by forming the firstprotective film 161 of stripe shape and etching the regions on bothsides of the first protective film to such a depth that reaches thelower cladding layer 5. Then a protective film made of ZrO₂ is formed onthe top surface and the side face of the ridge, and on the surfaces onboth sides thereof which are exposed by etching. The wafer is thendipped in hydrofluoric acid thereby to remove the first protective film161 by lift-off. Then similarly to the first embodiment, the resonanceend face and the electrodes are formed thereby to obtain the laserdevice which has only the first waveguide region C₁ with the sectionalstructure as shown in FIG. 9. In the second comparative embodiment, thestripe ridge is formed by etching to such a depth as thickness of 0.2 μmof the p-type cladding layer remains on both sides of the ridgesimilarly to the first waveguide region C₁ of the first comparativeembodiment.

The laser device thus obtained has shorter service life than that of thefirst embodiment since the stripe is formed by etching deeper than theactive layer, and does not make a practically useful laser device withthe service life as short as shown in FIG. 12.

The laser device of the present invention has the first waveguide regionC₁ and the second waveguide region C₂ as the waveguide in the resonatordirection, and therefore provides excellent device reliability andcontrollability of transverse mode. The present invention also provideslaser devices of various device characteristics with simple designmodifications.

While it has been difficult to achieve excellent device characteristicsof conflicting items such as practical level of device reliability andstable oscillation in the transverse mode at the same time, the laserdevice of the present invention combines excellent productivity,reliability and device characteristics. Moreover, it is made possible toobtain laser beams of various spot shapes and various aspect ratios byproviding the first waveguide region C₁ partially on the light emittingside of the resonance end face. Thus the present invention is capable ofachieving various beam characteristics and has a great effect ofexpanding the range of applications of laser device.

In the nitride semiconductor laser device of the prior art, satisfactoryyield of production and productivity can be achieved only with stripedlaser device because of the difficulty in the regrowth of crystal and inthe implantation of ion such as proton. When the active layer havingnitride semiconductor which includes In, significant damage is causedand the service life of the device decreases significantly, andtherefore only the effective refractive index type laser device could beselected. In contrast, the laser device of the present invention has thefirst waveguide region C₁ and the second waveguide region C₂ andtherefore achieves controllability of transverse mode and excellent beamcharacteristics while ensuring reliability of the device. Also thedevice structure allows manufacturing with high yield of production evenin volume production and makes it possible to apply and drasticallyproliferate the nitride semiconductor laser device. Moreover, when usedas the light source for an optical disk system of high recordingdensity, such an excellent laser device can be provided that is capableof operation over 1000 hours with 30 mW of output power and aspect ratioin a range from 1.0 to 1.5 without shift of transverse mode in theranges of output power for both reading data (5 mW) and writing data (30mW).

1. A semiconductor laser device comprising a laminate structureconsisting of a semiconductor layer of first conductivity type, anactive layer and a semiconductor layer of second conductivity type,which is different from the first conductivity type, that are stacked inorder, said laminate structure having a waveguide region to guide alight in a direction perpendicular to the direction of width, saidwaveguide region being formed by restricting the light from spreading inthe direction of width in the active layer and in the proximity thereof,wherein the waveguide region has a first waveguide region and a secondwaveguide region, the first waveguide region being a region where lightis confined within the limited active layer by means of a difference inthe refractive index between the active layer and the regions on bothsides of the active layer by limiting the width of the active layer, andthe second waveguide region being a region where the light is confinedtherein by providing effective difference in refractive index in theactive layer.
 2. The semiconductor laser device according to claim 1;wherein said first waveguide region has the active layer of which widthis restricted by forming a first ridge so as to include the activelayer, wherein said second waveguide region constituted by including aregion having effectively higher refractive index caused by forming asecond ridge in the layer of the second conductivity type.
 3. Thesemiconductor laser device according to claim 2; wherein said firstridge is formed by etching both sides of the first ridge until the layerof the first conductivity type is exposed and said second ridge isformed by etching both sides of the second ridge so that a part of thelayer of the second conductivity type remains on the active layer. 4.The semiconductor laser device according to claim 3; wherein a thicknessof the layer of the second conductivity type located on the active layeron both sides of the second ridge is 0.1 μm or less.
 5. Thesemiconductor laser device as in one of claims 2-4; wherein said secondridge is longer than said first ridge.
 6. The semiconductor laser deviceas in one of claims 1-5; wherein said first waveguide region includesone resonance end face of the laser resonator.
 7. The semiconductorlaser device according to claim 6; wherein said one resonance end faceis a light emitting face.
 8. The semiconductor laser device as in one ofclaims 1-7; wherein a length of said first waveguide region is 1 μm ormore.
 9. The semiconductor laser device as in one of claims 1-8; whereinsaid semiconductor layer of the first conductivity type, said activelayer and said semiconductor layer of the second conductivity type areformed from nitride semiconductor respectively.
 10. The semiconductorlaser device as in one of claims 1-9; wherein said active layer isconstituted from a nitride semiconductor layer which includes In. 11.The semiconductor laser device as in one of claims 1-10; furthercomprising insulation films on both sides of said first ridge and onboth sides of said second ridge, said insulation films being made of amaterial selected from the group consisting of oxides of Ti, V, Zr, Nb,Hf and Ta and compounds SiN, BN, SiC and AlN.
 12. A semiconductor laserdevice comprising; a laminate structure being consisted of a layer ofthe first conductivity type, an active layer and a layer of the secondconductivity type that is different from the first conductivity typebeing stacked in order, said laminate structure being provided with astripe waveguide region, wherein said stripe waveguide region has atleast a first waveguide region C₁ in which a stripe-shaped, waveguidebased on absolute refractive index and a second waveguide region C₂ inwhich a stripe-shaped waveguide based on effective refractive index,which are arranged in the direction of the resonator.
 13. Thesemiconductor laser device according to claim 12; wherein the absoluterefractive index of said first waveguide region C₁ is achieved by meansof the stripe ridge which is provided so as to include the layer of thefirst conductivity type, the active layer and the layer of the secondconductivity type, and the effective refractive index of said secondwaveguide region C₂ is achieved by means of the stripe ridge which isprovided in the layer of second conductivity type.
 14. A semiconductorlaser device comprising a laminate structure including a layer of thefirst conductivity type, an active layer and a layer of the secondconductivity type that is different from the first conductivity typebeing stacked in order, said laminate structure being provided with awaveguide region of stripe configuration, wherein said stripe waveguideregion has at least a second waveguide region where a portion of thelayer of the second conductivity type is removed and a stripe ridge isprovided in the layer of the second conductivity type, and a firstwaveguide region C₁ where portions of the layer of second conductivitytype, the active layer and the layer of second conductivity type areremoved and a stripe ridge is provided in the layer of the firstconductivity type, which are arranged in the direction of resonator. 15.The semiconductor laser device as in one of claims 12-14; wherein saidfirst waveguide region and said second waveguide region are constitutedby removing a part of the laminate structure and forming a ridgewaveguide comprising a stripe ridge.
 16. The semiconductor laser deviceas in one of claims 12-15; wherein said a length of the second waveguideregion is longer than said first waveguide region.
 17. The semiconductorlaser device as in one of claims 12-16; wherein at least one of theresonance end faces of the semiconductor laser device is formed at theend of the first waveguide region.
 18. The semiconductor laser deviceaccording to claim 17; wherein a resonance end face formed on the end ofthe first waveguide region C₁ is a light emitting face.
 19. Thesemiconductor laser device as in one of claims 17, 18; wherein a lengthof the first waveguide region which has the resonance end face on theend face thereof is 1 μm or longer.
 20. The semiconductor laser deviceas in one of claims 12-19; wherein said semiconductor layer of the firstconductivity type, said active layer and said semiconductor layer of thesecond conductivity type are formed from nitride semiconductorrespectively.
 21. The semiconductor laser device according to claim 20;wherein said active layer is constituted from a nitride semiconductorlaser which includes In.
 22. The semiconductor laser device as in one ofclaims 20, 21; wherein said semiconductor layer of the firstconductivity type include n-type nitride semiconductor and saidsemiconductor layer of the second conductivity type include p-typenitride semiconductor.
 23. The semiconductor laser device according toclaim 22; wherein said second waveguide region has a p-type claddinglayer which includes p-type nitride semiconductor and the stripe ridgeof the second waveguide region is formed while keeping the thickness ofthe p-type cladding layer is less than 0.1 μm.
 24. The semiconductorlaser device as in one of claims 20-23; wherein a side faces of thestripe ridge of the first waveguide region and a side faces of thestripe ridge of the second waveguide region are exposed, and aninsulation film is provided on the side face of the stripe ridge, saidinsulation film being made of a material selected from the groupconsisting of oxides of at least one element selected from Ti, V, Zr,Nb, Hf and Ta and at least one kind of compounds SiN, BN, SiC and AlN.25. The semiconductor laser device as in one of claims 20-24; wherein awidth of said stripe ridge is in a range from 1 μm to 3 μm.
 26. A methodfor manufacturing the semiconductor laser device comprising; alaminating process in which the layer of the first conductivity type,the active layer and the layer of the second conductivity type, arestacked in order by using nitride semiconductor to form a laminatestructure, a process of forming a first protective film of stripeconfiguration after forming the laminate structure, a first etchingprocess in which the laminate structure is etched in a portion thereofwhere the first protective film is not formed thereby to form the striperidge in the layer of the second conductivity type, a second etchingprocess in which a third protective film is formed via the firstprotective film on a portion of the surface which has been exposed inthe first etching process and the laminate is etched in a portionthereof where the third protective film is not formed thereby to formthe stripe ridge in the layer of first conductivity type, a process inwhich a second protective film having insulating property made of amaterial different from the first protective film is formed on the sideface of the stripe ridge and on the nitride semiconductor surfaceexposed by etching, and a process of removing the first protective filmafter the second protective film has been formed.